SWCommandProcessor: fix the CPReg structure fields

This structure fields should match byte-to-byte the layout of MMIO registers:
it is addressed using the MMIO reg address when doing a CP MMIO read. This was
unfortunately not the case, causing CP reads to be mostly broken with the
software renderer.
Merge pull request #27 from delroth/swcp-struct-fix

SWCommandProcessor: fix the CPReg structure fields