drm/radeon/kms/combios: verify dac_adj values are valid

Some vbios dac_adj tables are all zeros.  Check for that
case and use the default table if so.

Should fix fdo bug 27478.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_combios.c

@@ -760,6 +760,8 @@ struct radeon_encoder_primary_dac *radeon_combios_get_primary_dac_info(struct
 			dac = RBIOS8(dac_info + 0x3) & 0xf;
 			p_dac->ps2_pdac_adj = (bg << 8) | (dac);
 		}
+		/* if the values are all zeros, use the table */
+		if (p_dac->ps2_pdac_adj)
 			found = 1;
 	}
 
@@ -895,6 +897,8 @@ struct radeon_encoder_tv_dac *radeon_combios_get_tv_dac_info(struct
 			bg = RBIOS8(dac_info + 0x10) & 0xf;
 			dac = RBIOS8(dac_info + 0x11) & 0xf;
 			tv_dac->ntsc_tvdac_adj = (bg << 16) | (dac << 20);
+			/* if the values are all zeros, use the table */
+			if (tv_dac->ps2_tvdac_adj)
 				found = 1;
 		} else if (rev > 1) {
 			bg = RBIOS8(dac_info + 0xc) & 0xf;
@@ -908,6 +912,8 @@ struct radeon_encoder_tv_dac *radeon_combios_get_tv_dac_info(struct
 			bg = RBIOS8(dac_info + 0xe) & 0xf;
 			dac = (RBIOS8(dac_info + 0xe) >> 4) & 0xf;
 			tv_dac->ntsc_tvdac_adj = (bg << 16) | (dac << 20);
+			/* if the values are all zeros, use the table */
+			if (tv_dac->ps2_tvdac_adj)
 				found = 1;
 		}
 		tv_dac->tv_std = radeon_combios_get_tv_info(rdev);
@@ -925,6 +931,8 @@ struct radeon_encoder_tv_dac *radeon_combios_get_tv_dac_info(struct
 				    (bg << 16) | (dac << 20);
 				tv_dac->pal_tvdac_adj = tv_dac->ps2_tvdac_adj;
 				tv_dac->ntsc_tvdac_adj = tv_dac->ps2_tvdac_adj;
+				/* if the values are all zeros, use the table */
+				if (tv_dac->ps2_tvdac_adj)
 					found = 1;
 			} else {
 				bg = RBIOS8(dac_info + 0x4) & 0xf;
@@ -933,6 +941,8 @@ struct radeon_encoder_tv_dac *radeon_combios_get_tv_dac_info(struct
 				    (bg << 16) | (dac << 20);
 				tv_dac->pal_tvdac_adj = tv_dac->ps2_tvdac_adj;
 				tv_dac->ntsc_tvdac_adj = tv_dac->ps2_tvdac_adj;
+				/* if the values are all zeros, use the table */
+				if (tv_dac->ps2_tvdac_adj)
 					found = 1;
 			}
 		} else {