drm/msm/dsi-staging: free roi command packet

Change frees dsi packet after sending the roi
commands everytime. Roi packet memory was getting
leaked before this change.

CRs-Fixed: 2375784
Change-Id: Ia1e708c23830673ebcd7cc1eafea3a13cbf4958f
Signed-off-by: Vara Reddy <varar@codeaurora.org>