i965/fs: Fix and document component().

i965/fs: Fix and document component().

This fixes a number of bugs of component() by reimplementing it in
terms of horiz_offset(): Handling of base registers starting at a
non-zero subreg_offset, handling of strided registers and overflow of
subreg_offset into reg_offset.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>