msm: msm_fb: Add range checks for fb cmap.

Add appropriate range checks to ensure valid access
through framebuffer cmap. This prevents illegal
access into memory.

Change-Id: I9e82a27a2d12f31d8150ea577514576824f8b1f5
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>