drm/dp_mst: Rename drm_dp_mst_is_dp_mst_end_device() to be less redundant

It's already prefixed by dp_mst, so we don't really need to repeat
ourselves here. One of the changes I should have picked up originally
when reviewing MST DSC support.

There should be no functional changes here

Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Sean Paul <seanpaul@google.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306234623.547525-2-lyude@redhat.com