drm/vc4: Increase the number of planes per crtc in FKMS.

The number assigned was arbitrary as one primary, one overlay,
and one cursor.
The number has to be below the DRM limit of 32 planes total,
and the current firmware API limit of 16 planes total.

Increase the number to 8 planes per crtc (1 primary,
6 overlay, and a cursor).

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