sfxge(4): support runtime VI window size

Medford2 uses a configurable VI window size, and requires
updates to register accesses to use a runtime VI window size
rather than the *_STEP register constants used for earlier
controllers.

Update the common code to query the VI window size via MCDI,
and add new EFX_BAR_VI_* accessor macros for per-VI registers.

The existing EFX_BAR_TBL_* macros can be used for non-VI
register tables (and for code that can never be called for
a Medford2 controller e.g. Siena-only code).

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18158