msm_fb: display: switch mdp mode without turning off timing generator

MDP need to be in idle state before switching to different mode.
Dmap_done interrupt is delivered when the last line of frame is
clocked out. Therefore mdp is in idle state during dmap_done ISR.
This patch execute mdp mode switching inside dmap_done ISR.
Therefore timing generator is not necessary to be turned off
since mdp is in idle state already.

CRs-fixed: 350154
Change-Id: Ib10557bde90856ffd5284a0a76dddbe92f2abc0e
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>