drm/i915/fbc: Extract helpers to compute FBC control register values

Declutter the *_fbc_activate() functions by pulling all the
control register value computations into helpers.

I left the enable bit in *_fbc_activate() in the hopes of maybe
using the helpers in the *_fbc_deactivate() paths as well instead
of the current rmw approach. That won't be possible at least
quite yet since we clobber the fbc->params before deactivating
FBC so we could end up changing some of the values live, which
given FBC's lack of/poor double buffering would likely not go
so well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-6-ville.syrjala@linux.intel.com


Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>