drm/i915/display: Avoid divide by zero

skl_crtc_allocate_plane_ddb allow iter.data_rate to be zero
which could cause divide by zero in skl_allocate_plane_ddb,
check against that.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250905104626.1274147-3-juhapekka.heikkila@gmail.com