drm/amd/pm: drop unnecessary feature->mutex lock protections(V2)

drm/amd/pm: drop unnecessary feature->mutex lock protections(V2)

As these operations are performed in hardware setup and there
is actually no race conditions during this period considering:
1. the hardware setup is serial and cannot be in parallel
2. all other operations can be performed only after hardware
   setup complete.

V2: rich the commit log description

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>