drm/i915: Silence __iomem sparse warn

We don't care about __iomem mismatch when dealing with error
pointers. Silence it with ERR_CAST().

drivers/gpu/drm/i915/display/intel_dpt.c:136:21: warning: incorrect type in assignment (different address spaces)
drivers/gpu/drm/i915/display/intel_dpt.c:136:21:    expected struct i915_vma *[assigned] vma
drivers/gpu/drm/i915/display/intel_dpt.c:136:21:    got void [noderef] __iomem *[assigned] iomem

v2: The code moved into intel_dpt.c
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712161815.24776-1-ville.syrjala@linux.intel.comReviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/display/intel_dpt.c

@@ -133,7 +133,7 @@ struct i915_vma *intel_dpt_pin(struct i915_address_space *vm)
 	iomem = i915_vma_pin_iomap(vma);
 	i915_vma_unpin(vma);
 	if (IS_ERR(iomem)) {
-		vma = iomem;
+		vma = ERR_CAST(iomem);
 		goto err;
 	}