drm/etnaviv: Fix error path on failure to enable bus clk

Since commit 65f037e8 ("drm/etnaviv: add support for slave interface
clock") the reg clock is enabled before the bus clock and we need to undo
its enablement on error.

Fixes: 65f037e8 ("drm/etnaviv: add support for slave interface clock")
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>