drm/amd/display: make hubp1_wait_pipe_read_start() static

It's a local function, let's make it static.

AGD: remove prototype in dcn10_hubp.h

Signed-off-by: Tales Lelo da Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>