cxl/region: Handle endpoint decoders in cxl_region_find_decoder()

In preparation for adding a new caller of cxl_region_find_decoders()
teach it to find a decoder from a cxl_endpoint_decoder structure.

Combining switch and endpoint decoder lookup in one function prevents
code duplication in call sites.

Update the existing caller.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Tested-by: Wonjae Lee <wj28.lee@samsung.com>
Link: https://lore.kernel.org/r/79ae6d72978ef9f3ceec9722e1cb793820553c8e.1706736863.git.alison.schofield@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>