drm/xe/fbdev: Extract intel_fbdev_fb_prefer_stolen()

drm/xe/fbdev: Extract intel_fbdev_fb_prefer_stolen()

Pull the "should we keep the bios fb in stolen?" logic into
into a helper function, same as was done for i915. Gives us
a single place where to tweak the heuristics.

v2: changes related to rebase and consolidated other conditions
    for the stolen preference into this single function (Vinod)

v3: avoid including intel_display_core.h (Jani Nikula)

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/20260220170908.201422-5-vinod.govindapillai@intel.com