drm/exynos/ipp: Validate buffer enqueue requests

drm/exynos/ipp: Validate buffer enqueue requests

As for now there is no validation of incoming buffer
enqueue request as far as the gem buffers are being
concerned. This might lead to some undesired cases
when the driver tries to operate on invalid buffers
(wiht no valid gem object handle i.e.).
Add some basic checks to rule out those potential issues.

Signed-off-by: Beata Michalska <b.michalska@samsung.com>
[mszyprow: rebased onto v3.19]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>