drm/exynos: Use unsigned long for possible_crtcs

Change all instances of possible_crtcs in the exynos drm driver to be
unsigned long. This matches the type used in the drm layer.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>