msm: mdss: restore dsi interrupt mask bits

DSI interrupts are not enabled at LK. For cont_splash
case, dsi interrupt mask bits need to be restored to
enable interrupts so that dcs command be sent to panel.

CRs-Fixed: 745369
Change-Id: Ice61d538387c5d37dd9e077f01b4c6d4593cc393
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>