drm/omap: fix NULL deref crash with SDI displays

Fix a NULL deref bug introduced in commit 24aac601

 ("drm: omapdrm:
sdi: Allocate the sdi private data structure dynamically").

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2f803bfc-3ffe-332a-7b9a-d59a39db4630@ti.com
Fixes: 24aac601

 ("drm: omapdrm: sdi: Allocate the sdi private data structure dynamically")
Reported-by: Tony Lindgren <tony@atomide.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>