drm/amdgpu: Group gfx sysfs functions

Make amdgpu_gfx_sysfs_init/fini functions as common entry points for all
gfx related sysfs nodes.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>