drm/msm: move wq handling to KMS code

The global workqueue is only used for vblanks inside KMS code. Move
allocation / flushing / deallcation of it to msm_kms.c

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/662573/


Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>