drm/amdgpu/display: enable CONFIG_DRM_AMD_DC_DCN1_0 by default

It's required for displays on Raven.  The DCN bandwidth calcs use
floating point, but DCN is APU only and it already depends on
X86.

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