Add speed feature to disable splitmv

Added a speed feature in speed 1 to disable splitmv for HD (>=720)
clips. Test result on stdhd set: 0.3% psnr loss and 0.07% ssim
loss. Encoding speedup is 36%.

(For reference: The test result on derf set showed 2% psnr loss
and 1.6% ssim loss. Encoding speedup is 34%. SPLITMV should be
enabled for small resolution videos.)

Change-Id: I54f72b94f506c6d404b47c42e71acaa5374d6ee6