drm/i915: Silence __iomem sparse warn

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.com
Reviewed-by: Matthew Auld <matthew.auld@intel.com>