drm/amdkfd: Add process eviction counters to sysfs

Add per-process eviction counters to sysfs to keep track of
how many eviction events have happened for each process.

v2: rename the stats dir, and track all evictions per process, per device.
v3: Simplify the stats kobject handling and cleanup.
v4: more code cleanup

Signed-off-by: Philip Cox <Philip.Cox@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>