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 03cea610 ("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: 03cea610 ("drm/i915/dsb: fix extra warning on error path handling")
Resolves: https://gitlab.freedesktop.org/drm/intel/issues/8Signed-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