agp/intel-gtt: Add intel_gmch_gtt_read_entry()

agp/intel-gtt: Add intel_gmch_gtt_read_entry()

i915 wants to read out the PTE(s) populated by the BIOS/GOP
to verify that the framebuffer is in the correct location.
Introduce intel_gmch_gtt_read_entry() that reads out the
PTE and decodes it to a somewhat abstract form. For now
we just return the dma_addr, present bit, and local memory
bit. I didn't bother with the snoop bit/etc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250313140838.29742-4-ville.syrjala@linux.intel.com
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>