disp: msm: sde: fix panel mode check during SUI validations

During SUI validations and transitions, the encoders
are iterated to find the panel-mode. If concurrent
writeback is enabled, it is assumed as video mode.
Add check to consider only the built-in display
panel mode during sui to avoid wrong interpretation.

Change-Id: I2901eb29a0799571a95f6ab6f9f3f2c6154424c5
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>