drm/vc4: Fix reading of frame count on GEN5 / Pi4

The frame count values moved within registers DISPSTAT1 and
DISPSTAT2 with GEN5, so update the accessor function to
accommodate that.

Fixes: b51cd7ad ("drm/vc4: hvs: Fix frame count register readout")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>