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

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

Following commit f773568b6ff8 ("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 <[email protected]>
References: f773568b6ff8 ("drm/i915: nuke the duplicated stolen discovery")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]