msm_fb: display: remove mutex from mipi d2l register read/write

Mutex had been acquired at dsi_on() already. Hence no need
to acquire same mutex at mipi d2l registers read/write function
again.

Change-Id: Ie47590771c287a8e7c42aae568c1703a7e9107a4
Signed-off-by: kuogee hsieh <khsieh@codeaurora.org>