build: simplify Meson subproject fallbacks

All of these projects use meson.override_dependency() so we can
stop referencing their internal variable name to grab the
depndencies we need.

backend/session/meson.build

@@ -1,6 +1,6 @@
 libseat = dependency('libseat',
 	version: '>=0.2.0',
-	fallback: ['seatd', 'libseat'],
+	fallback: 'seatd',
 	default_options: ['server=disabled', 'man-pages=disabled'],
 )
 wlr_files += files('session.c')

backend/wayland/meson.build

@@ -1,5 +1,5 @@
 wayland_client = dependency('wayland-client',
-	fallback: ['wayland', 'wayland_client_dep'],
+	fallback: 'wayland',
 	default_options: wayland_project_options,
 )
 wlr_deps += wayland_client

meson.build

@@ -100,13 +100,13 @@ internal_features = {
 wayland_project_options = ['tests=false', 'documentation=false']
 wayland_server = dependency('wayland-server',
 	version: '>=1.20',
-	fallback: ['wayland', 'wayland_server_dep'],
+	fallback: 'wayland',
 	default_options: wayland_project_options,
 )
 
 drm = dependency('libdrm',
 	version: '>=2.4.109',
-	fallback: ['libdrm', 'ext_libdrm'],
+	fallback: 'libdrm',
 	default_options: [
 		'libkms=false',
 		'intel=false',

protocol/meson.build

@@ -1,6 +1,6 @@
 wayland_protos = dependency('wayland-protocols',
 	version: '>=1.24',
-	fallback: ['wayland-protocols', 'wayland_protocols'],
+	fallback: 'wayland-protocols',
 	default_options: ['tests=false'],
 )
 wl_protocol_dir = wayland_protos.get_variable('pkgdatadir')