Include xf86drm.h instead of sarea.h

Fixes build against current xorg-x11-server-devel from Fedora
34/35/rawhide.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
12 jobs for xf86drm.h in 1 minute and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #9235825
container-build

00:00:14

 
  Build
passed #9235828
xserver-1.13

00:00:43

passed #9235829
xserver-1.14

00:00:47

passed #9235830
xserver-1.15

00:00:42

passed #9235831
xserver-1.16

00:00:47

passed #9235832
xserver-1.17

00:00:43

passed #9235833
xserver-1.18

00:00:47

passed #9235834
xserver-1.18-clang

00:00:47

passed #9235835
xserver-1.19

00:00:47

passed #9235836
xserver-1.19-clang

00:00:47

passed #9235826
xserver-1.20

00:00:56

passed #9235827
xserver-1.20-clang

00:00:49