drm/xe: Fix possible exec queue leak in exec IOCTL

In a couple of places after an exec queue is looked up the exec IOCTL
returns on input errors without dropping the exec queue ref. Fix this
ensuring the exec queue ref is dropped on input error.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: <stable@vger.kernel.org>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241105043524.4062774-2-matthew.brost@intel.com