Despite what the HVS documentation indicates, the VC4 does not actually
support SAND tiling modes for any RGB format and only semiplanar YUV420
formats (NV12/NV21) can be used in these tiling modes.

The driver currently claims to support RGB formats for the associated
modifiers, so remove them from the supported list in the
format_mod_supported helper for RGB formats.

Remove further checks that are no longer necessary along the way, since
semi-planar YUV420 formats support every SAND tiling mode.
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181214141218.12671-1-paul.kocialkowski@bootlin.com