drm/radeon: Ensure radeon bo is unreserved in radeon_gem_va_ioctl

Found with lockdep while testing gpu reset.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Matthew Dawson <matthew@mjdsystems.ca>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>