fbmem: Remove mutex use from do_fb_ioctl()

Remove use of this mutex to allow multiple msm_fb ioctl calls.
Mutexes within the individual fb_ioctl calls already provide
the necessary locking.

This is based on android-msm-2.6.35 commit:
a809819d (msm_fb: display: Add delay kickoff to MDDI)
which includes this change among others.

Change-Id: Iee68bf3556ab913f916868988f4adbad1ed6b2a0
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>