drm/vc4: Add support for gamma on BCM2711

BCM2711 changes from a 256 entry lookup table to a 16 point
piecewise linear function as the pipeline bitdepth has increased
to make a LUT unwieldy.

Implement a simple conversion from a 256 entry LUT that userspace
is likely to expect to 16 evenly spread points in the PWL. This
could be improved with curve fitting at a later date.

Co-developed-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>