drm/exynos: 'win' is always unsigned

The index for the hardware layer is always >=0. Previous
code that also used -1 as special index is now gone.

Also apply this to 'ch_enabled' (decon/fimd), since the
variable is on the same line (and is again always unsigned).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>