drm/amd/display: fix MPO HUBP underflow with Scatter Gather

[why]
With Scatter Gather enabled, HUBP underflows during MPO enabled video
playback. hubp_init has a register write that fixes this problem, but
the register is cleared when HUBP gets power gated.

[how]
Make a call to hubp_init during enable_plane, so that the fix can
be applied after HUBP powers back on again.

Signed-off-by: Zi Yu Liao <ziyu.liao@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>