msm_fb: display: provision of mixer reset function

Mdp has hardware bug which cause fetch engine stalled if less than
3 pixels at begin or end of line be cropped into destination. Reset
mixer can fix this problem. However frame work can provide better
work around solution by falling back to gpu if the condition detected.
This patch will not reset mixer but provide mixer reset function in
case of mixer reset is need in future.

CRs-fixed: 443018
Change-Id: I38ee591f28bf1b81f4fbd58838281b43ba0a8498
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>