drm/i915/cmdparser: Use explicit goto for error paths

In the next patch we will be adding a second valid
termination condition which will require a small
amount of refactoring to share logic with the BB_END
case.

Refactor all error conditions to jump to a dedicated
exit path, with 'break' reserved only for a successful
parse.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Chris Wilson <chris.p.wilson@intel.com>