drm/exynos: cleanup name of gem object for exynos_drm

Struct of gem object in exynos_drm driver is struct exynos_drm_gem_obj.
It's too long and we can know its meaning of name without _obj postfix.

We use several names to variable name of gem object for exynos_drm -
exynos_gem_obj, gem_obj and obj. Especially "obj" name can cause
misunderstanding with variable name "obj" of struct drm_gem_object.

This will clean about name of gem object for exynos_drm as follows.
s/struct exynos_drm_gem_obj/struct exynos_drm_gem
s/exynos_gem_obj or gem_obj or obj/exynos_gem

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>