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: Shirish S <shirish.s@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>