drm/amd/display: Calculate DSC number of slices in debugfs when forced

[why]
When comparing current DSC timing settings with enforced through
debugfs we have to calculate number of both vertical and horisontal
slices. So instead of doing that every time we should just
use number of slices rather than setting its dimensions.

[how]
In connector's dsc preferred settings structure change slice height
and slice width parameters to number of slices vertical and horisontal.
Also calculate number of slices in debugfs rather in create_stream_for_sink.

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>