drm/amdgpu: fix pstate setting issue

drm/amdgpu: fix pstate setting issue

[WHY]
0, original pstate X
1, ctx_A_create -> ctx_A->stable_pstate = X
2, ctx_A_set_pstate (Y) -> current pstate is Y (PEAK or STANDARD)
3, ctx_B_create -> ctx_B->stable_pstate =  Y
4, ctx_A_destroy -> restore pstate to X
5, ctx_B_destroy -> restore pstate to Y
Above sequence will cause final pstate is wrong (Y), should be original X.

[HOW]
When ctx_B create,
if  ctx_A touched pstate setting
(not auto, stable_pstate_ctx != NULL),
set ctx_B->stable_pstate the same value as ctx_A saved,
if stable_pstate_ctx == NULL,
fetch current pstate to fill
ctx_B->stable_pstate.

Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org