drm/i915/display: reduce includes in g4x_dp.h includes

Only include what's needed.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e68c71ad1c1c1de7f5bc659edeb208818cddde72.1667383630.git.jani.nikula@intel.com