drm/amdgpu: Don't default to DC support for Kaveri and older

We've had a number of users report failures to detect and light up
display with DC with LVDS and VGA. These connector types are not
currently supported with DC. I'd like to add support but unfortunately
don't have a system with LVDS or VGA available.

In order not to cause regressions we should probably fallback to the
non-DC driver for ASICs that support VGA and LVDS.

These ASICs are:
 * Bonaire
 * Kabini
 * Kaveri
 * Mullins

ASIC support can always be force enabled with amdgpu.dc=1

v2: Keep Hawaii on DC
v3: Added Mullins to the list

Cc: stable@vger.kernel.org
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -2158,10 +2158,18 @@ bool amdgpu_device_asic_has_dc_support(enum amd_asic_type asic_type)
 	switch (asic_type) {
 #if defined(CONFIG_DRM_AMD_DC)
 	case CHIP_BONAIRE:
-	case CHIP_HAWAII:
 	case CHIP_KAVERI:
 	case CHIP_KABINI:
 	case CHIP_MULLINS:
+		/*
+		 * We have systems in the wild with these ASICs that require
+		 * LVDS and VGA support which is not supported with DC.
+		 *
+		 * Fallback to the non-DC driver here by default so as not to
+		 * cause regressions.
+		 */
+		return amdgpu_dc > 0;
+	case CHIP_HAWAII:
 	case CHIP_CARRIZO:
 	case CHIP_STONEY:
 	case CHIP_POLARIS10: