drm/ati_pcigart: Fix error code in drm_ati_pcigart_init()

The drm_ati_pcigart_init() function was originally suppose to return one
on success and zero on failure, but these days it returns a mix of
zero, one and -ENOMEM on failure.

This patch cleans it up and modifies the caller so now the function
returns zero on success and negative error codes on failure.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181217070344.GC12159@kadam