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>

drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c

@@ -177,7 +177,6 @@ void calc_rc_params(struct rc_params *rc, enum colour_mode cm, enum bits_per_com
 {
 	float bpp_group;
 	float initial_xmit_delay_factor;
-	int source_bpp;
 	int padding_pixels;
 	int i;
 
@@ -217,8 +216,6 @@ void calc_rc_params(struct rc_params *rc, enum colour_mode cm, enum bits_per_com
 			rc->initial_xmit_delay++;
 	}
 
-	source_bpp = MODE_SELECT(bpc * 3, bpc * 2, bpc * 1.5);
-
 	rc->flatness_min_qp     = ((bpc == BPC_8) ?  (3) : ((bpc == BPC_10) ? (7)  : (11))) - ((minor_version == 1 && cm == CM_444) ? 1 : 0);
 	rc->flatness_max_qp     = ((bpc == BPC_8) ? (12) : ((bpc == BPC_10) ? (16) : (20))) - ((minor_version == 1 && cm == CM_444) ? 1 : 0);
 	rc->flatness_det_thresh = 2 << (bpc - 8);