drm: Trust format_mod_supported() when it OKs a plane modifier.

For parameterized modifiers (Broadcom's SAND and UIF), we need to
allow the parameter fields to be filled in, while exposing only the
variant of the modifier with the parameter unfilled in the internal
arrays and the format blob.

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180316220435.31416-1-eric@anholt.net


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>