MLK-19021: drm/imx/dcss: fix RTRM clock reference

MLK-19021: drm/imx/dcss: fix RTRM clock reference

RTRM clock reference was saved in apb_clk, instead of rtrm_clk. Hence,
when blanking and clocks go off, APB clock counter was 2, instead of 1.

Because IRQ_STEER controller uses APB clock as well, the APB clock ref
counter will never go to 0. Unless DCSS is never used.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>