Add a tiling atttribute to the modifier descriptor, which let's us
get the tiling without listing the modifiers twice.

v1-v2: Unchanged.
v3:
- Initialize .tiling to I915_TILING_NONE explicitly (Ville)
- Move from previous patch lookup_modifier() to here, where it's first
  used.

Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020195138.1841242-4-imre.deak@intel.com