ui: do not build-depend or link with libdrm, it is not needed

Opengl support brings up libdrm. But actually nothing uses this library
or includes any of its headers. Just remove checking for it from configure.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180630165448.30795-1-mjt@msgid.tls.msk.ru
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>