drm/i915: Fix i915_dma_map_single positive error code

i915_dma_map_single relies on dma_mapping_error, which returns positive
error codes. Found by static checker.

Introduced by commit 678d96fb
("drm/i915: Track GEN6 page table usage").

v2: Return negative error code and renamed commit title. (Dan)
v3: Missing reported-by tag (Daniel)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>