drm/etnaviv: Don't ignore errors on getting clocks

There might be good reasons why the getting a clock failed. To treat the
clocks as optional we're specifically only interested in ignoring -ENOENT,
and devm_clk_get_optional() does just that.

Note that this preserves the original behavior of all clocks being
optional. The binding document mandates the "bus" clock while the dove
machine only specifies "core".

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>