msm_fb: display: extend mdp debugfs with force blt enable

Enabling/disabling mixer0 or mixer1 blt mode is a critical way to
debug display issues, so add debugfs interfaces to force blt mode
enabled/disabled with adb shell commands.

CRs-fixed: 390953
Change-Id: I96a568f22398867571a796a863c098f749f5618b
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>