drm/connector: hdmi: Do atomic check when necessary

It's ok to pass atomic check successfully if an atomic commit tries to
disable the display pipeline which the connector belongs to. That is,
when the crtc or the best_encoder pointers in struct drm_connector_state
are NULL, drm_atomic_helper_connector_hdmi_check() should return 0.
Without the check against the NULL pointers, drm_default_rgb_quant_range()
called by drm_atomic_helper_connector_hdmi_check() would dereference
the NULL pointer to_match in drm_match_cea_mode().

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Call trace:
 drm_default_rgb_quant_range+0x0/0x4c (P)
 drm_bridge_connector_atomic_check+0x20/0x2c
 drm_atomic_helper_check_modeset+0x488/0xc78
 drm_atomic_helper_check+0x20/0xa4
 drm_atomic_check_only+0x4b8/0x984
 drm_atomic_commit+0x48/0xc4
 drm_framebuffer_remove+0x44c/0x530
 drm_mode_rmfb_work_fn+0x7c/0xa0
 process_one_work+0x150/0x294
 worker_thread+0x2dc/0x3dc
 kthread+0x130/0x204
 ret_from_fork+0x10/0x20

Fixes: 8ec116ff ("drm/display: bridge_connector: provide atomic_check for HDMI bridges")
Fixes: 84e541b1 ("drm/sun4i: use drm_atomic_helper_connector_hdmi_check()")
Fixes: 65548c8f ("drm/rockchip: inno_hdmi: Switch to HDMI connector")
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250110084821.3239518-2-victor.liu@nxp.com


Signed-off-by: Maxime Ripard <mripard@kernel.org>