drm/amd/amdgpu: cleanup coding style a bit

There is DEVICE_ATTR mechanism in separate attribute define.
So this change is to use attr array, also use
sysfs_create_files in init function & sysfs_remove_files in
fini function.
This maybe make the code a bit readable.

Signed-off-by: Bernard Zhao <bernard@vivo.com>

Changes since V1:
*Use DEVICE_ATTR mechanism

Link for V1:
*https://lore.kernel.org/patchwork/patch/1228076/



V2: make array const to fix build errors

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>