drm/amdgpu: remove va cursors for all mappings

drm/amdgpu: remove va cursors for all mappings

va_cursor struct needs to be cleaned even if the mapping
has been removed already.

Also simplify it by make it a void function as return value
check isn't needed as its called during tear down.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>