drm/exynos: separeated fimd_power_on into some parts.

Changelog v2:
fix pm operation when resume.

Changelog v1:
this patch separetes fimd_power_on into fimd_activate and fimd_clock and
fimd_activate function will call fimd_clock to control fimd power and
vsync interrupt.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>