drm/amd/display: Fix number of slices not being checked for dsc

[why]
num_slices_h was not being checked

[How]
Fix the typo and check num_slices_h

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>