drm/i915: Use intel_ggtt_read_entry() in the BIOS FB takeover

Use intel_ggtt_read_entry() instead of open coding the PTE
read/decode in the BIOS FB takeover code. So far this codepath
only covers platforms with LMEMBAR.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250313140838.29742-6-ville.syrjala@linux.intel.com


Reviewed-by: Jouni Högander <jouni.hogander@intel.com>