msm_fb: display: add option to switch mdp operating mode

Currently, mdp switch operating mode without turning off
timing generator. This patch adds option to have mdp switch
operating mode (direct out and writeback) with timing generator
turned off based on configuration of panel file.

CRs-fixed: 405987
Change-Id: I92057757863218222d8647d33d57f6de9b3b3765
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>