msm: mdss: perform proper clock handoff during cont. splash screen

During continuous splash screen need to ensure clocks are properly ref
counted once and released once handoff has happened. Doing it in
different places can lead to improper order and cause clocks to be
disabled. Fix this by centralizing this into footswitch ctrl which is
called when continous splash screen is enabled.

Change-Id: I17f3d5517d9a0583221fdb5c354bf1992ddeddbd
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Pawan Kumar <pavaku@codeaurora.org>
msm: mdss: implement overlay prepare ioctl

Overlay prepare will allow a full frame with multiple overlays to be
configured at once and perform frame level checks before being pushed to
display. This function guarantees that if the call is successful, the
frame can be displayed successfully on the screen when display commit is
called.

Change-Id: I276b6fe3e0a872a2e93170f3cea1002d3ce0dac9
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Anusha Koduru <kanusha@codeaurora.org>