drm/i915: Determine DRRS frontbuffer_bits ahead of time

Pre-determine the frontbuffer_bits for the each pipe during
intel_drrs_enable(). Will become useful for bigjoiner use cases
soon.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315132752.11849-5-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>