drm/i915/dsb: fix cmd_buf being wrongly set

drm/i915/dsb: fix cmd_buf being wrongly set

The "err" label is not really "err", but rather "out" since the return
path is shared between error condition and normal path. This broke when
commit 794003c6c961 ("drm/i915/dsb: fix extra warning on error path
handling") added a "dsb->cmd_buf = NULL;" there, making DSB to stop
working since now all writes would pass-through via mmio.

Remove the set to NULL since it's actually not needed: we only set it if
all steps are successful. While at it, rename the label so this confusion
doesn't happen again.

Fixes: 794003c6c961 ("drm/i915/dsb: fix extra warning on error path handling")
Resolves: https://gitlab.freedesktop.org/drm/intel/issues/8
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127221119.384754-1-lucas.demarchi@intel.com