drm/amdgpu/pm: Fix NULL pointer dereference when get power limit

Because powerplay_table initialization is skipped under
sriov case, We check and set default lower and upper OD
value if powerplay_table is NULL.

Fixes: 7968e974 ("drm/amdgpu/pm: Fix the power1_min_cap value")
Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reported-by: Yin Zhenguo <zhenguo.yin@amd.com>
Suggested-by: Lazar Lijo <lijo.lazar@amd.com>
Suggested-by: Alex Deucher <Alexander.Deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org