drm/msm: add kernel mapping of writeback framebuffer

Add kernel mapping of writeback framebuffer to facilitate
viewing of image framebuffer using debugger tools.

Enable kernel mapping by:

echo 1 > /sys/kernel/debug/dri/0/Virtual-1/fb_kmap

CRs-Fixed: 2009714
Change-Id: I26395f26532858bc6a8bfef2321be76deed69dd2
Signed-off-by: Alan Kwong <akwong@codeaurora.org>