drm/i915: Verify the BIOS FB first PTE on non-LMEMBAR systems

drm/i915: Verify the BIOS FB first PTE on non-LMEMBAR systems

Now that we have intel_ggtt_read_entry() we can easily read out the
first PTE of the BIOS FB and verify that it looks correct. We'll
also use the extracted dma address to figure out where in stolen
the FB lives (so far we've just assumed that it sits at offset 0,
and in practice that does seem to be true, but better safe than
sorry).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250313140838.29742-7-ville.syrjala@linux.intel.com
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>