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)
Status Job ID Name Coverage
  Build
passed #46263
xserver-1.13

00:00:29

passed #46264
xserver-1.14

00:00:29

passed #46265
xserver-1.15

00:00:29

passed #46266
xserver-1.16

00:00:28

passed #46267
xserver-1.17

00:00:28

passed #46268
xserver-1.18

00:00:28

passed #46269
xserver-1.18-clang

00:00:32

passed #46270
xserver-1.19

00:00:28

passed #46271
xserver-1.19-clang

00:00:31

passed #46261
xserver-1.20

00:00:42

passed #46262
xserver-1.20-clang

00:00:34