drm/rockchip: vop: split out core clock enablement into separate functions

Judging from the iommu code, both the hclk and aclk are necessary for
register access. Split them off into separate functions from the regular
vop enablement, so that we can use them elsewhere as well.

Fixes: d0b912bd

 ("iommu/rockchip: Request irqs in rk_iommu_probe()")
[prerequisite change for the actual fix]
Cc: stable@vger.kernel.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180612132028.27490-2-heiko@sntech.de