drm/amdgpu: Prefer dev_warn over printk

Fix the below warning:

WARNING: Prefer [subsystem eg: netdev]_warn([subsystem]dev, ... then
dev_warn(dev, ... then pr_warn(...  to printk(KERN_WARNING ...

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>