drm/i915: Split the smem and lmem plane readout apart

Declutter initial_plane_vma() a bit by pulling the lmem and smem
readout paths into their own functions.

TODO: the smem path should still be fixed to get and validate
      the dma address from the pte as well

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/20240202224340.30647-12-ville.syrjala@linux.intel.com