package/mesa3d: gbm needs a DRI driver or a Gallium driver w/ EGL

package/mesa3d: gbm needs a DRI driver or a Gallium driver w/ EGL

src/gbm/cd6bfad@@gbm at sha/main_backend.c.o: In function `_gbm_create_device':
backend.c:(.text+0x38): undefined reference to `gbm_dri_backend'
backend.c:(.text+0x40): undefined reference to `gbm_dri_backend'
backend.c:(.text+0x74): undefined reference to `gbm_dri_backend'
backend.c:(.text+0x78): undefined reference to `gbm_dri_backend'
collect2: error: ld returned 1 exit status

This issue has been trigged since [1]:
"package/mesa3d: add option to configure gbm support"

Before the patch, the gbm support was autodetected by meson and enabled
only when at least one dri driver was enabled [2].

On the Buildroot side, the gbm support was explicitely enabled only when
BR2_PACKAGE_MESA3D_OPENGL_EGL was set.

We have two cases:
- At least one DRI driver.
- No DRI driver but one Gallium w/ EGL enable (EGL selected or not by the
  Gallium driver). In this case the meson build system set with_dri to true
  (even if no DRI driver is enabled) to use the builtin:egl_dri2 [3].

The gbm's meson build system seems to handle the case where no dri driver is
enabled [4] but it still use main/backend.c source file [6] that use
gbm_dri_backend [7]. So with_dri2 must always be set.

Probably a missing check in meson.build:

 if with_gbm and not with_dri
   error('GBM backend needs a dri driver or a gallium driver w/ EGL support.')
 endif

Add a dependency on GBM option:

 depends on BR2_PACKAGE_MESA3D_DRI_DRIVER \
         || (BR2_PACKAGE_MESA3D_GALLIUM_DRIVER && BR2_PACKAGE_MESA3D_OPENGL_EGL)

Fixes:
http://autobuild.buildroot.net/results/b9b6281983388dc22d929887d653da3db60f1f2c

[1] b6c051acf787c804e732bc58ba8d7e440701a168
[2] https://gitlab.freedesktop.org/mesa/mesa/blob/19.3/meson.build#L348
[3] https://gitlab.freedesktop.org/mesa/mesa/blob/19.3/meson.build#L212
[4] https://gitlab.freedesktop.org/mesa/mesa/blob/19.3/src/gbm/meson.build#L37
[5] https://gitlab.freedesktop.org/mesa/mesa/blob/19.3/src/gbm/meson.build#L24
[6] https://gitlab.freedesktop.org/mesa/mesa/blob/19.3/src/gbm/main/backend.c#L38
[7] http://lists.busybox.net/pipermail/buildroot/2020-February/274425.html

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[yann.morin.1998@free.fr: fix dependency of comment]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>