drm/vc4: hdmi: Fix off by ones in vc4_hdmi_read/write()

The variant->registers[] has ->num_registers elements so the >
comparison needs to be changes to >= to prevent an out of bounds
access.

Fixes: 311e305f ("drm/vc4: hdmi: Implement a register layout abstraction")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910100748.GA79916@mwanda