i965: Split struct brw_reg out from brw_eu.h into its own header.

i965: Split struct brw_reg out from brw_eu.h into its own header.

struct brw_instruction and the related instruction emitting code won't
be useful on Gen8+, as the instruction encoding changed.  However, the
struct brw_reg code is still extremely valuable.

While we're at it, fix up some style points:
- s/GLuint/unsigned/g
- s/GLint/int/g
- s/GLshort/int16_t/g
- s/GLushort/uint16_t/g
- s/INLINE/inline/g
- Replace tabs with spaces
- Put return types on a separate line from the function name/parameters
- Remove trailing whitespace
- Remove extraneous whitespace around function parameters

Reviewed-by: Eric Anholt <eric@anholt.net>