disp: msm: dsi: add missing dsi ctrl mutex lock in host timing update

Acquire dsi_ctrl->ctrl_lock lock before programming dsi ctrl
registers. Failing this may lead to race conditions in register
programming.
Add missing mutex lock inside dsi_ctrl_host_timing_update().

Change-Id: Ic86cbe282333c0b4d63ae3d5b3356a5d24752203
Signed-off-by: Rajeev Nandan <quic_rajeevny@quicinc.com>