cxl: Replace HDM decoder granularity magic numbers

When reviewing the CFMWS parsing code that deals with the HDM decoders,
I noticed a couple of magic numbers. This commit replaces these magic numbers
with constants defined by the CXL 3.0 specification.

v2:
 - Change references to CXL 3.0 specification (David)
 - CXL_DECODER_MAX_GRANULARITY_ORDER -> CXL_DECODER_MAX_ENCODED_IG (Dan)

Signed-off-by: Adam Manzanares <a.manzanares@samsung.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20220829220249.243888-1-a.manzanares@samsung.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>