drm/amd/display: Remove set but not used variables 'h_ratio_chroma', 'v_ratio_chroma'

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

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dwb_scl.c: In function dwb_program_horz_scalar:
drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dwb_scl.c:725:11: warning: variable h_ratio_chroma set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dwb_scl.c: In function dwb_program_vert_scalar:
drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dwb_scl.c:806:11: warning: variable v_ratio_chroma set but not used [-Wunused-but-set-variable]

They are not used since commit 345429a6 ("drm/amd/display:
Add DCN2 DWB")

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>