drm/i915: Fix error handling for cursor/sprite plane create failure

drm/i915: Fix error handling for cursor/sprite plane create failure

intel_cursor_plane_create() and intel_sprite_plane_create() return
an error pointer, so let's not mistakenly look for a NULL pointer.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
References: https://lists.freedesktop.org/archives/intel-gfx/2016-November/110690.html
Fixes: 28b12cf6134b ("drm/i915: Bail if plane/crtc init fails")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478550057-24864-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>