backend/drm: add hardcoded hwdata path fallback

Add hardcoded fallback "/usr/share/hwdata/pnp.ids" as a
temporary solution to get things working in Guix until
hwdata ships with pkg-config file.

diff --git a/backend/drm/meson.build b/backend/drm/meson.build
index 6fcb2c15..ed95360a 100644
--- a/backend/drm/meson.build
+++ b/backend/drm/meson.build
@@ -1,8 +1,7 @@
 hwdata = dependency(
 	'hwdata',
-	required: 'drm' in backends,
+	required: false,
 	native: true,
-	not_found_message: 'Required for the DRM backend.',
 )
 
 libdisplay_info = dependency(
@@ -19,15 +18,21 @@ libliftoff = dependency(
 	required: false,
 )
 
-if not (hwdata.found() and libdisplay_info.found() and features['session'])
+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
+
+if not (libdisplay_info.found() and features['session'])
 	subdir_done()
 endif
 
-hwdata_dir = hwdata.get_variable(pkgconfig: 'pkgdatadir')
 pnpids_c = custom_target(
 	'pnpids.c',
 	output: 'pnpids.c',
-	input: files(hwdata_dir / 'pnp.ids'),
+	input: pnp_ids,
 	feed: true,
 	capture: true,
 	command: files('gen_pnpids.sh'),