drm/amd/display: Fix a typo when computing dsc configuration

[why]
Remove a backslash symbol accidentally left in increase bpp function
when computing mst dsc configuration.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>