drm/i915/dmc: use intel uncore functions for forcewake register access

Move away from I915_READ_FW() and I915_WRITE_FW() and switch to using
intel_uncore_read_fw() and intel_uncore_write_fw(), respectively.

No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123140004.14136-1-jani.nikula@intel.com