clk: imx6: refine hdmi_isfr's parent to make HDMI work on i.MX6 SoCs w/o VPU

On i.MX6 SoCs without VPU (in my case MCIMX6D4AVT10AC), the hdmi driver
fails to probe:

[    2.540030] dwhdmi-imx 120000.hdmi: Unsupported HDMI controller
(0000:00:00)
[    2.548199] imx-drm display-subsystem: failed to bind 120000.hdmi
(ops dw_hdmi_imx_ops): -19
[    2.557403] imx-drm display-subsystem: master bind failed: -19

That's because hdmi_isfr's parent, video_27m, is not correctly ungated.
As explained in commit 5ccc248c ("ARM: imx6q: clk: Add support for
mipi_core_cfg clock as a shared clock gate"), video_27m is gated by
CCM_CCGR3[CG8].

On i.MX6 SoCs with VPU, the hdmi is working thanks to the
CCM_CMEOR[mod_en_ov_vpu] bit which makes the video_27m ungated whatever
is in CCM_CCGR3[CG8]. The issue can be reproduced by setting
CCMEOR[mod_en_ov_vpu] to 0.

Make the HDMI work in every case by setting hdmi_isfr's parent to
mipi_core_cfg.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>