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>