drm/imx: parallel-display: Adjust bus_flags handling

The bus_flags handling logic does not seem to cover all potential
usecases. Specifically, this seems to fail with an "edt,etm0700g0edh6"
display attached to an 24bit display interface, with interface-pix-fmt
= "rgb24" set in DT.

This patch fixes the problem by overriding the imx_crtc_state->bus_flags
from the imxpd->bus_flags only if the DT property "interface-pix-fmt" is
present or if the DI provides no formats.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>