drm/vram-helper: Integrate drm_gem_vram_init() into drm_gem_vram_create()

The drm_gem_vram_create() function is the only caller of the internal
helper drm_gem_vram_init(). Streamline the code by putting all code into
the create function.

v2:
	* rebased onto latest drm-tip
	* fixed typo in commit message (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200922145700.25413-2-tzimmermann@suse.de