drm/i915/gtt: Tidy up duplicate branches in gen8_gmch_probe()

Following commit f773568b ("drm/i915: nuke the duplicated stolen
discovery"), the if-else-chain for determining the GTT size is redundant
with the !chv branches all being the same.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
References: f773568b ("drm/i915: nuke the duplicated stolen discovery")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503212956.3948-1-chris@chris-wilson.co.uk