drm/exynos: gem: Simplify access to exynos GEM objects

Replace all calls to exynos_drm_gem_get_{dma_addr,size}, by a simpler
function exynos_drm_gem_get(). This lets the caller to get access to
exynos_drm_gem object and extract any information about GEM object
without searching object tree for getting each parameter.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>