drm/exynos: fix tests for valid FIMD window number

Valid values for FIMD windows are from 0 to WINDOWS_NR-1
inclusive (5 windows in total). The WINDOWS_NR is also
a size of fimd_context.win_data array.
However, early-return tests for wrong values of windows
accepted a value of WINDOWS_NR which is out of bound
for fimd_context.win_data.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>