meson: add wrap for libdrm

Sometimes as a developer when a new version of libdrm is required, it's
nice to not have to install it somewhere and set up pkg-confing, and
just let Meson do it for us.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10364>

meson.build

@@ -1657,7 +1657,11 @@ foreach d : _libdrm_checks
   if d[1]
     set_variable(
       'dep_libdrm_' + d[0],
-      dependency('libdrm_' + d[0], version : '>=' + _drm_ver)
+      dependency(
+        'libdrm_' + d[0],
+        version : '>=' + _drm_ver,
+        allow_fallback: true,
+      )
     )
   endif
 endforeach
@@ -1665,7 +1669,9 @@ endforeach
 with_gallium_drisw_kms = false
 if system_has_kms_drm
   dep_libdrm = dependency(
-    'libdrm', version : '>=' + _drm_ver,
+    'libdrm',
+    version : '>=' + _drm_ver,
+    allow_fallback: true,
     # GNU/Hurd includes egl_dri2, without drm.
     required : (with_dri2 and host_machine.system() != 'gnu') or with_dri3
   )