meson: Print a message about why a libdrm version was selected

We require a single version of libdrm for all of our libdrm
dependencies (core and driver), but the way this is structured can make
the error message less than helpful, as one driver might be the one
setting the libdrm requirement, while another might be the one that
generates the version failure.

This adds a simple message to the output announcing which libdrm module
set the version, which might be more helpful.

v2: - Use message suggested by Eric Engstrom

Fixes: c445b1d56f
       ("meson: Use the same version for all libdrm checks")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

meson.build

@@ -1121,12 +1121,17 @@ endif
 
 # Loop over the enables versions and get the highest libdrm requirement for all
 # active drivers.
+_drm_blame = ''
 foreach d : _libdrm_checks
   ver = get_variable('_drm_@0@_ver'.format(d[0]))
   if d[1] and ver.version_compare('>' + _drm_ver)
     _drm_ver = ver
+    _drm_blame = d[0]
   endif
 endforeach
+if _drm_blame != ''
+  message('libdrm @0@ needed because @1@ has the highest requirement'.format(_drm_ver, _drm_blame))
+endif
 
 # Then get each libdrm module
 foreach d : _libdrm_checks