drm/vc4: crtc: Move the HVS gamma LUT setup to our init function

Since most of the HVS channel is setup in the init function, let's move the
gamma setup there too. As this makes the HVS mode_set function empty, let's
remove it in the process.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d439da8f1592a450a6ad35ab1f9e77def17c7965.1599120059.git-series.maxime@cerno.tech