kmssink: fix tmp_kmsmem leaks

configure_mode_setting() keeps a ref on tmp_kmsmem which is released in
gst_kms_sink_show_frame().
But if for some reason configure_mode_setting() is re-called before
showing a frame or if none is showed this memory was leaked.
6 jobs for kmssink-leak in 37 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #66795
gst indent

00:01:12

passed #66794
manifest

00:00:24

 
  Build
passed #66796
build fedora x86_64

00:05:32

 
  Test
passed #66797
check fedora

00:14:50

 
  Full Builds
passed #66798
build android arm64 api28

00:04:43

passed #66799
build cerbero fedora x86_64

00:15:50