drivers/amd/pm: check the return value of amdgpu_bo_kmap

amdgpu_bo_kmap() returns error when fails to map buffer object. Add the
error check and propagate the error.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Li Zhong <floridsleeves@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>