drm/amd/display: Add scoped mutexes for amdgpu_dm_dhcp

[Why]
Guards automatically release mutex when it goes out of scope making
code easier to follow.

[How]
Replace all use of mutex_lock()/mutex_unlock() with guard(mutex).

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>