drm/exynos/vidi: simplify fake edid handling

Avoid assigning fake_edid_info to ctx->raw_edid. Always keep
ctx->raw_edid either an allocated pointer or NULL. Defer fake_edid_info
handling to .get_modes().

This should be functionally equivalent but slightly easier to follow.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>