drm/amd/display: Reduce DC chattiness

Log DC init but default log level to 0 (default for
amdgpu_dc_log) otherwise. Bug reporters can still make
DC more chatty by using the dc_log module param.
    amdgpu.dc_log = 1

v2: Only provide runtime option, no compile time config

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