fb: Flatten control flow in fb_set_var

Instead of wiring almost everything down to the very last line using
goto soup (but not consistently, where would the fun be otherwise)
drop out early when checks fail. This allows us to flatten the huge
indent levels to just 1.

Aside: If a driver doesn't set ->fb_check_var, then FB_ACTIVATE_NOW
does nothing. This bug exists ever since this code was extracted as a
common helper in 2002, hence I decided against fixing it. Everyone
just better have a fb_check_var to make sure things work correctly.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Peter Rosin <peda@axentia.se>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-28-daniel.vetter@ffwll.ch