drm/msm/dsi-staging: destroy dsi error_wq after disabling interrupts

Destroy dsi error_wq workqueue after disabling dsi interrupts.
We will hit NULL pointer dereference, when any dsi irq fires
after destroying error_wq and before disabling dsi interrupts.

CRs-Fixed: 2297926
Change-Id: I47cff928f7e2271a64f3c38209431e697a9e664e
Signed-off-by: Vara Reddy <varar@codeaurora.org>