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

commit c6a8de64f219a35b4757f1eecc6c2a234527b1d7 from
https://nv-tegra.nvidia.com/r/3rdparty/canonical/linux-jammy.git

 jetson_36.3

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


(cherry picked from commit e566507b)
Change-Id: Id6852afeb0fd5b215aeb4a957123bc24110b5659
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/3rdparty/canonical/linux-jammy/+/2985865


Reviewed-by: svcacv <svcacv@nvidia.com>
Reviewed-by: Jonathan Hunter <jonathanh@nvidia.com>
GVS: Gerrit_Virtual_Submit <buildbot_gerritrpt@nvidia.com>
Signed-off-by: Rajeshkumar Ramasamy <rajeshkumar.ramasamy@windriver.com>