drm/i915/rps: Prefer REG_FIELD_GET in intel_rps_get_cagf

Instead of masks/shifts settle on REG_FIELD_GET as the standard way to
extract reg fields. This allows future patches touching this code to also
consistently use REG_FIELD_GET and friends.

Suggested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114123348.3474216-2-badal.nilawar@intel.com