drm/vc4: Limit SAND tiling support to semiplanar YUV420 formats

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