drm/vc4: Enable gamma block only when required.

With HVS5 the gamma block is now only reprogrammed with
a disable/enable. Loading the table from vc4_hvs_init_channel
(called from vc4_hvs_atomic_enable) appears to be at an
invalid point in time and so isn't applied.

Switch to enabling and disabling the gamma table instead. This
isn't safe if the pipeline is running, but it isn't now.
For HVS4 it is safe to enable and disable dynamically, so
adopt that approach there too.

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