drm/amd/amdgpu/amdgpu_doorbell_mgr: Correct misdocumented param 'doorbell_index'

Fixes the following W=1 kernel build warning(s):

drivers/gpu/drm/amd/amdgpu/amdgpu_doorbell_mgr.c:123: warning: Function parameter or member 'doorbell_index' not described in 'amdgpu_doorbell_index_on_bar'
drivers/gpu/drm/amd/amdgpu/amdgpu_doorbell_mgr.c:123: warning: Excess function parameter 'db_index' description in 'amdgpu_doorbell_index_on_bar'

Reviewed-by: Shashank Sharma <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_doorbell_mgr.c

···
  *
  * @adev: amdgpu_device pointer
  * @db_bo: doorbell object's bo
- * @db_index: doorbell relative index in this doorbell object
+ * @doorbell_index: doorbell relative index in this doorbell object
  *
  * returns doorbell's absolute index in BAR
  */