drm/msm/sde: retain memory for display ram dump feature

DRM display module releases the splash buffer memory after
first frame trigger from client. For display ram dump feature
bootloader needs memory to show image in ramdump mode. This
change retains memory for ramdump display.

Change-Id: I6d587bceba3a35e81090786a3772f06298f6bd36
Signed-off-by: Narendra Muppalla <NarendraM@codeaurora.org>