drm/i915/selftests: Add mock selftest for remapped vmas

drm/i915/selftests: Add mock selftest for remapped vmas

Extend the rotated vma mock selftest to cover remapped vmas as
well.

TODO: reindent the loops I guess? Left like this for now to
ease review

v2: Include the vma type in the error message (Chris)
v3: Deal with trimmed sg
v4: Drop leftover debugs

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190509122159.24376-3-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>