drm/amdkfd: Separate pinned BOs destruction from general routine

Currently, all kfd BOs use same destruction routine. But pinned
BOs are not unpinned properly. Separate them from general routine.

v2 (Felix):
Add safeguard to prevent user space from freeing signal BO.
Kunmap signal BO in the event of setting event page error.
Just kunmap signal BO to avoid duplicating the code.

Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>