drm/amd/pm: fix and simplify workload handling

smu->workload_mask is IP specific and should not be messed with in
the common code. The mask bits vary across SMU versions.

Move all handling of smu->workload_mask in to the backends and
simplify the code.  Store the user's preference in smu->power_profile_mode
which will be reflected in sysfs.  For internal driver profile
switches for KFD or VCN, just update the workload mask so that the
user's preference is retained.  Remove all of the extra now unused
workload related elements in the smu structure.

v2: use refcounts for workload profiles
v3: rework based on feedback from Lijo
v4: fix the refcount on failure, drop backend mask
v5: rework custom handling
v6: handle failure cleanup with custom profile
v7: Update documentation

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Kenneth Feng <kenneth.feng@amd.com>
Cc: Lijo Lazar <lijo.lazar@amd.com>