drm/i915/gt: Remove GEN12_SFC_DONE_MAX from register defs header

We shouldn't really be keeping track of how many SFC_DONE registers
our platforms can have, but rather how many SFC hardware units there can
be (each SFC unit will have one corresponding SFC_DONE register).  So
drop the stray GEN12_SFC_DONE_MAX definition we had in the register
definition file and replace it with an I915_MAX_SFC that follows the
pattern we use for other hardware units.  Note that our hardware has a
2:1:1 ratio of VD:VE:SFC, and as far as we know that pattern should
carry forward to future platforms, so we'll define it as #VCS/2.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311062835.163744-1-matthew.d.roper@intel.com