drm/amd/display: remove set but not used variable 'bpc'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c: In function get_pbn_from_timing:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:2364:11: warning:
 variable bpc set but not used [-Wunused-but-set-variable]

It is not used since commit e49f6936 ("drm/amd/display: use
proper formula to calculate bandwidth from timing"), this also
remove get_color_depth(), which is only used here.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>