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>

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c

@@ -934,6 +934,9 @@ void dcn20_enable_plane(
 	/* enable DCFCLK current DCHUB */
 	pipe_ctx->plane_res.hubp->funcs->hubp_clk_cntl(pipe_ctx->plane_res.hubp, true);
 
+	/* initialize HUBP on power up */
+	pipe_ctx->plane_res.hubp->funcs->hubp_init(pipe_ctx->plane_res.hubp);
+
 	/* make sure OPP_PIPE_CLOCK_EN = 1 */
 	pipe_ctx->stream_res.opp->funcs->opp_pipe_clock_control(
 			pipe_ctx->stream_res.opp,