i965/fs: Factor out universally broken calculation of the register component size.

i965/fs: Factor out universally broken calculation of the register component size.

This in principle simple calculation was being open-coded in a number
of places (in a series I haven't yet sent for review there will be a
couple more), all of them were subtly broken in one way or another:
None of them were handling the HW_REG case correctly as pointed out by
Connor, and fs_inst::regs_read() was handling the stride=0 case rather
naively.  This patch solves both problems and factors out the
calculation as a new fs_reg method.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>