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 <[e-mail]>
Reviewed-by: Lyude Paul <[e-mail]>
Signed-off-by: Lyude Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c

···
 // SPDX-License-Identifier: GPL-2.0-only
 /*
- * Test cases for for the DRM DP MST helpers
+ * Test cases for the DRM DP MST helpers
  */
 
 #define PREFIX_STR "[drm_dp_mst_helper]"