msm: mdss: avoid any hdmi update during cont splash screen handoff

While continuous splash screen is in progress need to ensure that other
display devices such as HDMI don't come in and modify state of clocks
and votes before handoff completed.

Change-Id: Ia85c4805b38080adecc5025affea37c392b11071
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>