egl/gbm: Fix config validation

egl/gbm: Fix config validation

In converting to shift/size-based validation, we lost a condition from
the ARGB/XRGB equivalence check, which left it working one way round
but not the other, and broke applications like glmark2-es2-drm on some
platforms. Restore the equivalent check that *both* configs actually
have an alpha channel before considering a mismatch.

Fixes: 7b4ed2b513ef ("egl: Convert configs to use shifts and sizes instead of masks")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>