drm/vc4: crtc: Restrict HACT_ACT setup to DSI

The HACT_ACT field only needs to be written to when using a DSI display.
Let's move that setup to our DSI branch to clear a bit the common path.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/7a93436f97666a2aa025686ef3ff3606de4bec67.1590594512.git-series.maxime@cerno.tech