drm/i915/vdsc: Fix first_line_bpg_offset calculation

On checking DSC1.1 Errata and DSC 1.2 spec the current formula
we were using was incorrect to calculate first_line_bpg_offset.
The new fixed formula is derived from C model.

--v2
-Use clamp function in linux/minmax.h [Ankit]

--v3
-remove linux/minmax.h header
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804083737.3844575-1-suraj.kandpal@intel.com

drivers/gpu/drm/i915/display/intel_vdsc.c

@@ -80,13 +80,19 @@ calculate_rc_params(struct drm_dsc_config *vdsc_cfg)
 	int bpc = vdsc_cfg->bits_per_component;
 	int bpp = vdsc_cfg->bits_per_pixel >> 4;
 	int qp_bpc_modifier = (bpc - 8) * 2;
+	int uncompressed_bpg_rate;
+	int first_line_bpg_offset;
 	u32 res, buf_i, bpp_i;
 
 	if (vdsc_cfg->slice_height >= 8)
-		vdsc_cfg->first_line_bpg_offset =
-			12 + DIV_ROUND_UP((9 * min(34, vdsc_cfg->slice_height - 8)), 100);
+		first_line_bpg_offset =
+			12 + (9 * min(34, vdsc_cfg->slice_height - 8)) / 100;
 	else
-		vdsc_cfg->first_line_bpg_offset = 2 * (vdsc_cfg->slice_height - 1);
+		first_line_bpg_offset = 2 * (vdsc_cfg->slice_height - 1);
+
+	uncompressed_bpg_rate = (3 * bpc + (vdsc_cfg->convert_rgb ? 0 : 2)) * 3;
+	vdsc_cfg->first_line_bpg_offset = clamp(first_line_bpg_offset, 0,
+						uncompressed_bpg_rate - 3 * bpp);
 
 	/*
 	 * According to DSC 1.2 spec in Section 4.1 if native_420 is set: