backend-drm: remove unnecessary ifdefs

Remove unnecessary ifdefs for DRM_MODE_CONNECTOR_DSI, DRM_MODE_CONNECTOR_DPI.
They are both provided by libdrm and were introduced long before 2.4.83 (the
lowest version we currently support).

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>
3 jobs for rm_unnecessary_ifdefs in 1 minute and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container Prep
passed #1007342
container_prep

00:00:17

 
  Build
passed #1007343
build-native-meson-default-options

00:00:54

passed #1007344
build-native-meson-no-gl-renderer

00:00:43