drm/etnaviv: fix tx clock gating on some GC7000 variants

commit 4bce2442 ("drm/etnaviv: disable tx clock gating for GC7000
rev6203") accidentally applied the fix for i.MX8MN errata ERR050226 to
GC2000 instead of GC7000, failing to disable tx clock gating for GC7000
rev 0x6023 as intended.

Additional clean-up further propagated this issue, partially breaking
the clock gating fixes added for GC7000 rev 6202 in commit 432f51e7
("drm/etnaviv: add clock gating workaround for GC7000 r6202").

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>