accel/qaic: Fix NULL pointer deref in qaic_destroy_drm_device()

If qaic_destroy_drm_device() is called before the device has fully
initialized it will cause a NULL pointer dereference as the drm device
has not yet been created. Fix this with a NULL check.

Fixes: c501ca23

 ("accel/qaic: Add uapi and core driver file")
Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Carl Vanderlip <quic_carlv@quicinc.com>
Reviewed-by: Pranjal Ramajor Asha Kanojiya <quic_pkanojiy@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230602210440.8411-3-quic_jhugo@quicinc.com