drm/amd/amdgpu: Convert messages in gmc v6 to dev_*()

Make the kernel log messaging more consistent and use the
dev_*() functions instead of a mix of DRM_*() and printk().

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>