mdss: fb: update smem_len when user updates parameters

There is an ioctl for userspace to update the variable
parameters. Frame buffer driver has to update the shared
memory length according to the new variable information.

Change-Id: Ic24ee3e4b1101b4e1ef4e9db2182602aa5175812
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>