drm/i915: convert INTEL_DISPLAY_ENABLED() into a function

There's no need for this to be a macro. Add some documentation too.

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/480b9b697b2ffa0c8677115bd443ba633801c6c1.1696336887.git.jani.nikula@intel.com