drm/mxsfb: improve clk handling for axi clk

Ignoring errors from devm_clk_get() is wrong. To handle not all platforms
having an axi clk use devm_clk_get_optional() instead and do proper error
handling.

Also the clk API handles NULL as a dummy clk (which is also returned by
devm_clk_get_optional() if there is no clk) so there is no need to check
for NULL before calling clk_prepare_enable() or its counter part.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720153254.18071-1-u.kleine-koenig@pengutronix.de