drm/i915: Extract intel_plane_needs_low_address()

drm/i915: Extract intel_plane_needs_low_address()

Extract the naked "gmch? -> need a low ggtt address" check into
a more descriptive helper (intel_plane_needs_low_address()).
The goal being to eliminate all display specific stuff from the
low level pinning code.

The actual implementation still abuses PIN_MAPPABLE to achieve
this goal. I'm not entire convinced that this whole thing even
needs to exist, and the original issue wasn't just caused by
some other bug. But no time to dig into it right now, so let's
keep going.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260416174448.28264-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>