mesa: use os_file_exists()

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
10 jobs for !1085 with osfile-exists in 10 minutes and 18 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #367625
debian

00:00:22

 
  Build+Test
passed #367627
meson-clang

00:09:22

passed #367631
meson-clover

00:09:55

passed #367630
meson-main

00:05:59

passed #367626
meson-swr-glvnd

00:08:06

passed #367632
meson-vulkan

00:02:25

passed #367634
scons-llvm

00:04:00

passed #367633
scons-nollvm

00:03:42

passed #367628
scons-swr

00:06:33

passed #367629
scons-win64

00:07:50