drm/i915: Move drrs hardware bit frobbing to small helpers

Split the drrs code that actually changes the refresh rate
(via PIPECONF or M/N values) to small helper functions that
only deal with the hardware details an nothing else. We'll
soon have a third way of doing this, and it's less confusing
when each difference method lives in its own funciton.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220127093303.17309-5-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>