drm/vc4: Disable Gamma control on HVS5 due to issues writing the table

Still under investigation, but the conditions under which the HVS
will accept values written to the gamma PWL are not straightforward.

Disable gamma on HVS5 again until it can be resolved to avoid
gamma being enabled with an incorrect table.

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