drm/amdgpu: Use rpm_mode flag instead of checking it again for rpm

Because the rpm_mode flag is already set when the driver
is initialized, we use it directly for runtime suspend/resume
instead of checking it again

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>