drm/i915/dp_mst: Handle error during DSC BW overhead/slice calculation

The MST branch device may not support the number of DSC slices a mode
requires, handle the error in this case.

Fixes: 4e0837a8 ("drm/i915/dp_mst: Account for FEC and DSC overhead during BW allocation")
Cc: stable@vger.kernel.org # v6.8+
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241009110135.1216498-1-imre.deak@intel.com