msm: mdss: Move splash thread functionality

framebuffer and overlay drivers are tightly coupled with
splash thread functionality. This adds lots of code
related to continuous splash screen handling in
both drivers. Moving them to separate file cleans up
both drivers and allows to handle continuous splash screen
cases efficiently.

CRs-fixed: 605934
Change-Id: Id89bb281dbbf22e726cf5fd97e93f891e5c73f60
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>