drm/amdgpu: missing bounds check in amdgpu_set_pp_force_state()

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: f3898ea12fc1 ('drm/amd/powerplay: add some sysfs interfaces for powerplay.')
Acked-by: Christian König <[email protected]>
Signed-off-by: Dan Carpenter <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>