drm/amd/display: Fix typo for helpers function name

[why]
Word "helper" was misspelled as "helpes" in
dm_helpes_dmub_outbox0_interrupt_control function.

[how]
Fix the spelling.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>