drm_fb_helper: Preserve capability to use atomic kms

Commit 5349ef31 (drm/fb: fix
FBIOGET/PUT_VSCREENINFO pixel clock handling) changed the logic of
when a pixclock was valid vs invalid.

The atomic kernel mode setting used by the kernel debugger relies upon
the drm_fb_helper_check_var() to always return -EINVAL.  Until a
better solution exists, this behavior will be restored.

CC: David Airlie <airlied@linux.ie>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>