drm/i915/fb: Don't store bitmasks in the intel_plane_caps enum

Variables of enum types can contain only the values listed at the enums
definition, so don't store bitmasks in intel_plane_caps enum variables.

Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.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/20211026161517.2694067-3-imre.deak@intel.com