drm/i915: Promote .format_mod_supported() to the lead role

Up to now we've used the plane's modifier list as the primary
source of information for which modifiers are supported by a
given plane. In order to allow auxiliary metadata to be embedded
within the bits of the modifier we need to stop doing that.

Thus we have to make .format_mod_supported() aware of the plane's
capabilities and gracefully deal with any modifier being passed
in directly from userspace.

v2: Rebase after NV12
    Simplify

Cc: Eric Anholt <eric@anholt.net>
References: https://lists.freedesktop.org/archives/dri-devel/2018-March/169782.html


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518162159.30305-1-ville.syrjala@linux.intel.com


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