ui: drop libdrm dependency.

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Merge remote-tracking branch 'remotes/kraxel/tags/ui-20180703-pull-request' into staging

ui: drop libdrm dependency.

# gpg: Signature made Tue 03 Jul 2018 10:18:36 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/ui-20180703-pull-request:
  ui: do not build-depend or link with libdrm, it is not needed

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -3883,7 +3883,7 @@ libs_softmmu="$libs_softmmu $fdt_libs"
 # opengl probe (for sdl2, gtk, milkymist-tmu2)
 
 if test "$opengl" != "no" ; then
-  opengl_pkgs="epoxy libdrm gbm"
+  opengl_pkgs="epoxy gbm"
   if $pkg_config $opengl_pkgs; then
     opengl_cflags="$($pkg_config --cflags $opengl_pkgs)"
     opengl_libs="$($pkg_config --libs $opengl_pkgs)"