drm/bochs: fix ptr_ret.cocci warnings

drivers/gpu/drm/bochs/bochs_mm.c:19:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Fixes: b3a25b9a ("drm/bochs: Convert bochs driver to VRAM MM")
CC: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: kbuild test robot <lkp@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190520122314.GA155389@lkp-kbuild22


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>