vc4: Clean up ->clock_select and is_dsi in vc4_crtc.c

[anholt]

danvet said about our clock_select detection in vc4_crtc.c:

Hm, so the usual way we solve the "crtc needs information from the encoder problem" is to add bits to the crtc state, and then fill those out in the encoders ->atomic_check function. In your case ->clock_select and is_dsi.

The benefit is mostly when you start doing hw readout (which is great even just to cross-check your modeset code), or when you need that information to check limits (which sooner or later tends to happen ime).

Let's try doing this.

[anholt]

WIP at https://github.com/anholt/linux/commits/drm-vc4-dsi-cleanup