drm/i915: s/phys_base/dma_addr/

The address we read from the PTE is a dma address, not a physical
address. Rename the variable to say so.

Reviewed-by: Andrzej Hajda <andrzej.hajda@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/20240202224340.30647-11-ville.syrjala@linux.intel.com