amdgpu/pm/legacy: fix suspend/resume issues

resume and irq handler happily races in set_power_state()

* amdgpu_legacy_dpm_compute_clocks() needs lock
* protect irq work handler
* fix dpm_enabled usage

v2: fix clang build, integrate Lijo's comments (Alex)

Closes: drm/amd#2524


Fixes: 3712e7a4 ("drm/amd/pm: unified lock protections in amdgpu_dpm.c")
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Tested-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name> # on Oland PRO
Signed-off-by: chr[] <chris@rudorff.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>