i965/disasm: Wrap opcode_desc look-up in a function.

i965/disasm: Wrap opcode_desc look-up in a function.

The function takes a device info struct as argument in addition to the
opcode number in order to disambiguate between multiple opcode_desc
entries for different instructions with the same opcode number.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> [v1]
[v2] mattst88: Put brw_opcode_desc() in brw_eu.c instead of moving it
               there in a later patch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> [v2]
[v3] mattst88: Return NULL if opcode >= ARRAY_SIZE(opcode_descs)
Reviewed-by: Matt Turner <mattst88@gmail.com>