drm/i915: Use 'enabled' instead of 'enable' consistently in sprite WM code

Let's be consistent and always call our variables 'enabled' insted of
the occasional 'enable'.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Spelling fix in the commit message, spotted by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>