drm/i915: Extract intel_fb_needs_cpu_access()

drm/i915: Extract intel_fb_needs_cpu_access()

Replace the naked "does the framebuffer have a clear color
plane?" checks with a more abstract helper that simply tells
us whether we require CPU access to the framebuffer's memory.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260416174448.28264-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>