drm/amd/display: fix missing-prototypes warning

[Why]
No declaration of hubp1_wait_pipe_read_start found in header file.

[How]
Add its declaration.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Becle Lee <becle.lee@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>