msm: mdss: change framebuffer split ratio at run time

This is to avoid kernel recompilation when a different
framebuffer split ratio is desired at run time. To
enable a new ratio, write the  desired value to
/sys/devices/virtual/graphics/fb0/msm_fb_split.
Changes will take effect only after client releases
all references to driver.

Change-Id: I4764399f80fc22ec5815a8e2818901189b3aaeb5
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>