HACK: drm/tidss: tidss_encoder: Force default format

This takes a leaf from drivers/gpu/drm/rockchip/rockchip_rgb.c

This is a bit of a chicken or hen issue at this point. when the call chain
looks like this:
tidss_atomic_check
drm_atomic_check_only
drm_atomic_commit
drm_client_modeset_commit_atomic
drm_client_modeset_commit_locked
drm_client_modeset_commit
drm_fb_helper_set_par
fbcon_init
visual_init

The bridge_attach is invoked, but bridge_enable is NOT invoked yet, at
this stage, we cannot really confirm the display format.

So, it begs the question - what do others do? why do they work?

Signed-off-by: Nishanth Menon <nm@ti.com>