drm/i915/irq: use intel de functions for forcewake register access

Move away from I915_READ_FW() and I915_WRITE_FW() in display code, and
switch to using intel_de_read_fw() and intel_de_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/20200123140004.14136-3-jani.nikula@intel.com