drm/simpledrm: Fix an NULL vs IS_ERR() bug

The devm_memremap() function doesn't return NULL, it returns error
pointers.

Fixes: 9a10c7e6 ("drm/simpledrm: Add support for system memory framebuffers")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/Y9JHzImRcUaa0mi1@kili