drm/i915/fbdev: Extract intel_fbdev_fb_prefer_stolen()

drm/i915/fbdev: Extract intel_fbdev_fb_prefer_stolen()

Consolidate the "should we allocate fbdev fb in stolen?"
check into a helper function. Makes it easier to change the
heuristics without having to change so many places.

v2: rebase related changes and consolidate all the prefer
    stolen conditions into a single function (Vinod)

v3: avoid including intel_display_core.h (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patch.msgid.link/20260220170908.201422-4-vinod.govindapillai@intel.com