The assumption in the upsampling code was that the crtc->gamma_size
size of the crtc's gamma table is a power of two. This is true for
almost all current driver + gpu combos at least on Linux, with typical
sizes of 256, 512, 1024 or 4096 slots.

However, Intel Gen-11 Icelake and later are outliers, as their gamma
table has 2^18 + 1 slots, very big and not a power of two!

Try to make upsampling behave at least reasonable: Replicate the
last gamma value to fill up remaining crtc->gamma_red/green/blue
slots, which would normally stay uninitialized. This is important,
because while the intel display driver does not actually use all
2^18+1 values passed as part of a GAMMA_LUT, it does need the
very last slot, which would not get initialized by the old code.

This should hopefully create reasonable behaviour with Icelake+
but is untested on the actual Intel hw due to lack of suitable
hw.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>