drm/amd/display: Remove set but not used variable 'source_bpp'

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

drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c: In function calc_rc_params:
drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c:180:6: warning: variable source_bpp set but not used [-Wunused-but-set-variable]

It is not used since commit 97bda032 ("drm/amd/display:
Add DSC support for Navi (v2)")

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>