drm: vc4: Add status of which display is updated through vblank

Previously multiple  displays were slaved off the same SMI
interrupt, triggered by HVS channel 1 (HDMI0).
This doesn't work if you only have a DPI or DSI screen (HVS channel
0), and gives slightly erroneous results with dual HDMI as the
events for HDMI1 are incorrect.

Use SMIDSW0 and SMIDSW1 registers to denote which display has
triggered the vblank.
Handling should be backwards compatible with older firmware.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>