drm/i915/ggtt: Add intel_ggtt_read_entry()

drm/i915/ggtt: Add intel_ggtt_read_entry()

The BIOS FB takeover code wants to read out the PTEs (or at least
one of them) to figure out where the FB is located in memory.
Currently we only do that for systems with LMEMBAR, and we've
open coded the PTE decoding in the display code. Introduce a more
proper abstract interface (intel_ggtt_read_entry()) for this purpose,
and implement it for all platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250313140838.29742-5-ville.syrjala@linux.intel.com
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>