egl/dri2: Guard against invalid fourcc formats

We already reject attempts to import images with invalid fourcc formats
but don't really guard the queries all that well.  This makes us error
out in any calls to eglQueryDmaBufModifiersEXT if the given format is
not a valid fourcc format.  We also add an assert to ensure that drivers
don't advertise any non-fourcc formats.

Cc: mesa-stable@lists.freedesktop.org
Tested-By: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 8c1b9882)