drm/i915: Do not WARN_ON with small framebuffers.

drm/i915: Do not WARN_ON with small framebuffers.

It's perfectly legal to create a fb with stride < 512, and one of
the kms_plane_scaling subtests creates a very small fb.

Downgrade the WARN_ON to a simple check check, and because this
function is potentially called on every atomic update/pageflip,
downgrade the other WARN_ON to a WARN_ON_ONCE, and do the right
thing here.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180116155331.75175-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>