drm/i915/gem: Fix the mman selftest

Using the I915_MMAP_TYPE_FIXED mmap type requires the TTM backend, so
for that mmap type, use __i915_gem_object_create_user() instead of
i915_gem_object_create_internal(), as we really want to tests objects
mmap-able by user-space.

This also means that the out-of-space error happens at object creation
and returns -ENXIO rather than -ENOSPC, so fix the code up to expect
that on out-of-offset-space errors.

Finally only use I915_MMAP_TYPE_FIXED for LMEM and SMEM for now if
testing on LMEM-capable devices. For stolen LMEM, we still take the
same path as for integrated, as that haven't been moved over to TTM yet,
and user-space should not be able to create out of stolen LMEM anyway.

v2:
 - Check the presence of the obj->ops->mmap_offset callback rather than
   hardcoding the supported mmap regions in can_mmap() (Maarten Lankhorst)

Fixes: 7961c5b6

 ("drm/i915: Add TTM offset argument to mmap.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210831122931.157536-1-thomas.hellstrom@linux.intel.com
(cherry picked from commit 450cede7

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>