msm_fb: display: detect if timing generator is on when do dma waiting

If the timing generator is not on, dma waiting won't return and system
crashes. So add TG on detection to avoid the issue.

CRs-fixed: 362503
Change-Id: Ic472e7b75be1fd8960e5ac18929181513085212e
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>