msm_fb: display: don't enable blt mode

The switch of blt mode may cause mdp hang on some targets. To avoid
blt switch, we won't allocate the pipe that requires blt
mode. Basically this implies two things: downscaling is done by
rotator and gpu composition is used in case of blt mode is needed.

Change-Id: I2d6f1a0e33a40af90e3a1a013a1b48b0df0c8ea2
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>