drm/amd/display: Prevent bandwidth overflow

[Why]
At very high pixel clock, bandwidth calculation exceeds 32 bit size
and overflow value. This causes the resulting selection of link rate
to be inaccurate.

[How]
Change order of operation and use fixed point to deal with integer
accuracy. Also address bug found when forcing link rate.
Signed-off-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -3444,10 +3444,13 @@ uint32_t dc_bandwidth_in_kbps_from_timing(
 {
 	uint32_t bits_per_channel = 0;
 	uint32_t kbps;
+	struct fixed31_32 link_bw_kbps;
 
 	if (timing->flags.DSC) {
-		kbps = (timing->pix_clk_100hz * timing->dsc_cfg.bits_per_pixel);
-		kbps = kbps / 160 + ((kbps % 160) ? 1 : 0);
+		link_bw_kbps = dc_fixpt_from_int(timing->pix_clk_100hz);
+		link_bw_kbps = dc_fixpt_div_int(link_bw_kbps, 160);
+		link_bw_kbps = dc_fixpt_mul_int(link_bw_kbps, timing->dsc_cfg.bits_per_pixel);
+		kbps = dc_fixpt_ceil(link_bw_kbps);
 		return kbps;
 	}