msm: msm_fb: Map RGB layers to the max possible size.

IOMMU Page Faults are observed on RGB layers during stability
because MDP is trying to fetch out of mapped area of the buffer.
This patch avoids the issue by overmapping the RGB layers to the
max possible RGB buffer size.

Change-Id: Idc7ac9acad70e351acac5dba2ae43b8731e84d35
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>