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>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hubp.h

@@ -790,5 +790,6 @@ bool hubp1_in_blank(struct hubp *hubp);
 void hubp1_soft_reset(struct hubp *hubp, bool reset);
 
 void hubp1_set_flip_int(struct hubp *hubp);
+void hubp1_wait_pipe_read_start(struct hubp *hubp);
 
 #endif