drm: logicvc: Fix uninitialized variable in probe

The "regmap" is supposed to be initialized to NULL but it's used
without being initialized.

Fixes: efeeaefe

 ("drm: Add support for the LogiCVC display controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/Yqh6OfSiPFuVrGo4@kili