drm/vc4: hvs: Fix frame count register readout

In order to get the field currently being output, the driver has been
using the display FIFO frame count in the HVS, reading a 6-bit field at
the offset 12 in the DISPSTATx register.

While that field is indeed at that location for the FIFO 1 and 2, the
one for the FIFO0 is actually in the DISPSTAT1 register, at the offset
18.

Fixes: e538092c ("drm/vc4: Enable precise vblank timestamping for interlaced modes.")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220331143744.777652-3-maxime@cerno.tech