drm/amdgpu: missing bounds check in amdgpu_set_pp_force_state()

There is no limit on high "idx" can go.  It should be less than
ARRAY_SIZE(data.states) which is 16.

The "data" variable wasn't declared in that scope so I shifted the code
around a bit to make it work.  Also I made "idx" unsigned.

Fixes: f3898ea1 ('drm/amd/powerplay: add some sysfs interfaces for powerplay.')
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>