intel/eu: Add more message descriptor helpers

intel/eu: Add more message descriptor helpers

We want to be able to extract data from descriptors as well as unify a
bit of the descriptor construction.

One of the unifications we do is to unify the read/write and dataport
descriptors.  On gen4-5, read/write are substantially different and the
read descriptors change between gen4 and gen4.x.  On gen6, they unified
layouts between read, write, and dataport.  Then, on gen8, they added
one bit to the message type field but left it reserved MBZ for
read/write messages.  This commit chooses to treat that as if they
expanded the field everywhere and just didn't have enough enum values
for read/write to bother with the extra bit.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>