backend/drm: require hwdata pkg-config file

The hardcoded fallback "/usr/share/hwdata/pnp.ids" was only a
temporary solution to get things working while distributions were
still working on shipping it.

backend/drm/meson.build

@@ -1,19 +1,19 @@
-if not features['session']
+hwdata = dependency(
+	'hwdata',
+	required: 'drm' in backends,
+	native: true,
+	not_found_message: 'Required for the DRM backend.',
+)
+
+if not (hwdata.found() and features['session'])
 	subdir_done()
 endif
 
-hwdata = dependency('hwdata', required: false, native: true)
-if hwdata.found()
 hwdata_dir = hwdata.get_variable(pkgconfig: 'pkgdatadir')
-	pnp_ids = files(hwdata_dir / 'pnp.ids')
-else
-	pnp_ids = files('/usr/share/hwdata/pnp.ids')
-endif
-
 pnpids_c = custom_target(
 	'pnpids.c',
 	output: 'pnpids.c',
-	input: pnp_ids,
+	input: files(hwdata_dir / 'pnp.ids'),
 	feed: true,
 	capture: true,
 	command: files('gen_pnpids.sh'),