Add argument description for the input argument
inst for amdgpu_device_ip_set_powergating_state.

Fixes the warning raised by the compiler:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:2182:
warning: Function parameter or struct member 'inst' not described in 'amdgpu_device_ip_set_powergating_state'

Cc: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>