drm/amdgpu: Prefer pr_err/_warn/_notice over printk in amdgpu_atpx_handler.c

Fixes the following style issues:

ERROR: open brace '{' following function definitions go on the next line
WARNING: printk() should include KERN_<LEVEL> facility level

Cc: Guchun Chen <guchun.chen@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Bert Karwatzki <spasswolf@web.de>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>