Skip gamma correction of cursor data if premultiplied R/G/B > alpha

The un-premultiplied R/G/B values would overflow the gamma LUT, so just
pass through the data unchanged, and leave it up to the HW how to
interpret such weird premultiplied alpha pixels.

Bugzilla: https://bugs.freedesktop.org/108355
11 jobs for cursor-unpremultiply-overflow-no-gamma in 2 minutes and 53 seconds (queued for 19 seconds)