backend-drm: make linear modifier to default

Make the linear modifier to the default of DRM format modifiers if
supported modifiers isn't get from kernel driver.
2 jobs for test in 1 minute and 14 seconds
Status Job ID Name Coverage
  Container Prep
passed #386595
container_prep

00:00:27

 
  Build
failed #386596
build-native-meson

00:00:47

 
Name Stage Failure
failed
build-native-meson Build
  RemovedInSphinx30Warning)
/root/.local/lib/python3.5/site-packages/sphinx_rtd_theme/search.html:20: RemovedInSphinx30Warning: To modify script_files in the theme is deprecated. Please insert a <script> tag directly in your theme instead.
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ninja: build stopped: cannot make progress due to previous errors.
Uploading artifacts...
build-*/meson-logs: found 2 matching files
prefix-*: found 54 matching files

Uploading artifacts to coordinator... ok
id=386596 responseStatus=201 Created token=ChoPsZKs
ERROR: Job failed: exit code 1