drm/display/selftests: drop extra word 'for' in comments for MST selftests

there is an unexpected word 'for' in the comments that need to be dropped

file - ./drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c
line - 3

* Test cases for for the DRM DP MST helpers

changed to:

* Test cases for the DRM DP MST helpers

Signed-off-by: Jiang Jian <jiangjian@cdjrlc.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220623100632.27056-1-jiangjian@cdjrlc.com