package/mesa3d: simplify the code enabling full-OpenGL

package/mesa3d: simplify the code enabling full-OpenGL

Currently, full-OpenGL is enabled by checking that at least one driver
is enabled. This is done by checking that the just-constructed lists of
drivers are not empty.

But we already have a variable that is set to 'y' as soon as a driver
is selected:

  - DRI drivers select BR2_PACKAGE_MESA3D_DRI_DRIVER

  - Gallium drivers select BR2_PACKAGE_MESA3D_GALLIUM_DRIVER

  - both BR2_PACKAGE_MESA3D_DRI_DRIVER and BR2_PACKAGE_MESA3D_GALLIUM_DRIVER
    select BR2_PACKAGE_MESA3D_DRIVER

So, BR2_PACKAGE_MESA3D_DRIVER is set to 'y' as soon as at least one
driver is selected. We can use that variable rather than comparing the
lists of drivers.

Also, rearrange the code in a more logical way, by moving the code that
enables OpenGL nearer to the code enabling EGL and GLES.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Bernd Kuhls <berndkuhls@hotmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>