drm/i915: Populate pipe dbuf slices more accurately during readout

During readout we cannot assume the planes are actually using the
slices they are supposed to use. The BIOS may have misprogrammed
things and put the planes onto the wrong dbuf slices. So let's
do the readout more carefully to make sure we really know which
dbuf slices are actually in use by the pipe at the time.

Cc: <stable@vger.kernel.org> # v5.14+
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220204141818.1900-2-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
(cherry picked from commit b3dcc6dc

)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>