drm/amd/display: fix null pointer deref when plugging in display

drm/amd/display: fix null pointer deref when plugging in display

[ Upstream commit 1f3b22e4eb162e0b1d423106a47484943a22a309 ]

[Why&How]
When system boots in headless mode, connecting a 4k display creates a
null pointer dereference due to hubp for a certain plane being null.
Add a condition to check for null hubp before dereferencing it.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>