drm/i915: Fix region start during initial plane readout

drm/i915: Fix region start during initial plane readout

On MTL the stolen region starts at offset 8MiB from the start of
LMEMBAR. The dma addresses are thus also offset by 8MiB. However the
mm_node/etc. is zero based, and i915_pages_create_for_stolen() will
add the appropriate region.start into the sg dma address. So when
we do the readout we need to convert the dma address read from
the PTE to be zero based as well.

Note that currently we don't take this path on MTL, but we should
and thus this needs to be fixed. For lmem this works correctly
already as the lmem region.start==0.

While at it let's also make sure the address points to somewhere within
the memory region. We don't need to check the size as
i915_gem_object_create_region_at() should later fail if the object size
exceeds the region size.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Tested-by: Paz Zcharya <pazz@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240202224340.30647-9-ville.syrjala@linux.intel.com