msm: msm_fb: remove mmio access through mmap

Disable access to mm io and add
appropriate range checks to ensure valid accesses
through framebuffer mmap. This prevents illegal
access into memory.

CRs-Fixed: 474706
Change-Id: If25166f2732433ef967e99c716440030b567aae9
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>