drm/amd/display: check plane state before validating fbc

While validation fbc, array_mode of the pipe is accessed
without checking plane_state exists for it.
Causing to null pointer dereferencing followed by
reboot when a crtc associated with external display(not
connected) is page flipped.

This patch adds a check for plane_state before using
it to validate fbc.
Signed-off-by: NShirish S <shirish.s@amd.com>
Reviewed-by: NRoman Li <Roman.Li@amd.com>
Reviewed-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c

@@ -1774,6 +1774,10 @@ static enum dc_status validate_fbc(struct dc *dc,
 	if (pipe_ctx->stream->sink->link->psr_enabled)
 		return DC_ERROR_UNEXPECTED;
 
+	/* Nothing to compress */
+	if (!pipe_ctx->plane_state)
+		return DC_ERROR_UNEXPECTED;
+
 	/* Only for non-linear tiling */
 	if (pipe_ctx->plane_state->tiling_info.gfx8.array_mode == DC_ARRAY_LINEAR_GENERAL)
 		return DC_ERROR_UNEXPECTED;