drm/amdkfd: Add eviction debug messages

Use WARN to print messages with backtrace when evictions are triggered.
This can help determine the root cause of evictions and help spot driver
bugs triggering evictions unintentionally, or help with performance tuning
by avoiding conditions that cause evictions in a specific workload.

The messages are controlled by a new module parameter that can be changed
at runtime:

  echo Y > /sys/module/amdgpu/parameters/debug_evictions
  echo N > /sys/module/amdgpu/parameters/debug_evictions

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>