drm/vc4: crtc: Use a shared interrupt

Some pixelvalves in vc5 use the same interrupt line so let's register our
interrupt handler as a shared one.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/5a915d374357f41083ac71779fa9b2c35a339c2f.1599120059.git-series.maxime@cerno.tech