drm/amd/amdgpu: Add missing kdoc 'inst' parameter in 'smu_dpm_set_power_gate' function

This commit adds the missing kdoc parameter descriptor for 'inst' in the
smu_dpm_set_power_gate function.

The 'inst' parameter, which specifies the instance of the IP block to
power gate/ungate.

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/amdgpu_smu.c:359: warning: Function parameter or struct member 'inst' not described in 'smu_dpm_set_power_gate'

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>