drm/msm/sde: avoid intf regs access in cont. splash

For continuous splash, driver needs to make sure not to
access some of the non-double buffered registers
during the first commit after continuous splash.
Enforce this by adding the necessary checks.

Change-Id: Iae8e1660374a0a9534a1840ffa200a6cb5e90144
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>