Only call drmmode_validate_leases if RandR is enabled

It would crash if RandR is disabled, e.g. because Xinerama is enabled.

Bugzilla: https://bugs.freedesktop.org/109230
(Ported from radeon commit b1c01698f577577e4a88bad0ae08fb5d998e7ebb)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
12 jobs for drmmode_validate_leases-no-randr in 1 minute and 32 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #78718
debian-testing

00:00:16

 
  Build
passed #78721
xserver-1.13

00:00:45

passed #78722
xserver-1.14

00:00:33

passed #78723
xserver-1.15

00:00:45

passed #78724
xserver-1.16

00:00:39

passed #78725
xserver-1.17

00:00:38

passed #78726
xserver-1.18

00:00:29

passed #78727
xserver-1.18-clang

00:00:32

passed #78728
xserver-1.19

00:00:29

passed #78729
xserver-1.19-clang

00:00:31

passed #78719
xserver-1.20

00:00:56

passed #78720
xserver-1.20-clang

00:00:42