drm/amd/display: fix a divided-by-zero error

[Why & How]

timing.dsc_cfg.num_slices_v can be zero and it is necessary to check
before using it.

This fixes the error "divide error: 0000 [#1] PREEMPT SMP NOPTI".

Reviewed-by: Aurabindo Pillai <Aurabindo.Pillai@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org