drm/vc4: Expose the format modifiers for firmware kms.

This should technically not expose VC4_T_TILED on pi4.  However, if we
don't expose anything, then userspace will assume that display can
handle whatever modifiers 3d can do (UIF on 2711).  By exposing a
list, that will get intersected with what 3D can do so that we get T
tiling for display on 2710 and linear on 2711.

Signed-off-by: Eric Anholt <eric@anholt.net>