mdss: fb: ensure that shared memory is available during probe

Populate shared memory length during probe for availability
to userspace through FSCREENINFO ioctl. Also, ensure that fb
length is always available.

Change-Id: I94d3f83098fbfd81a7aa20f6fc1f19e756aebc4e
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>