package/mesa3d: Fix dri checks

package/mesa3d: Fix dri checks

"MESA3D_EGL_PLATFORMS = drm" is only needed when dri drivers are
enabled:
http://cgit.freedesktop.org/mesa/mesa/tree/configure.ac?h=10.6#n1653

Unconditionally enable dri support, also for gallium drivers, quoting
http://cgit.freedesktop.org/mesa/mesa/tree/configure.ac?h=10.6#n1382

 # Strictly speaking libgbm does not require --enable-dri, although
 # both of its backends do. Thus one can build libgbm without any
 # backends if --disable-dri is set.
 # To avoid unnecessary complexity of checking if at least one backend
 # is available when building, just mandate --enable-dri.
   AC_MSG_ERROR([gbm requires --enable-dri])

Fixes
http://autobuild.buildroot.net/results/799/799f505594f668154af38cb2c2a1ed8a90337e9f/
http://autobuild.buildroot.net/results/d00/d00fe349e8710a1ad557507cf04b006216806868/
http://autobuild.buildroot.net/results/c7b/c7ba08c8cf54b2e7cf28c5e3454e92a408e724db/
http://autobuild.buildroot.net/results/cd7/cd717c9a3a1b0080192e7e765d990cce55513005/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>