exynos-drm: FIMD mode valid check can't use crtc display mode values

We can't use the crtc_ values here because those are only available
at the point when the mode is applied to a CRTC.
Use the standard values. They will always be the same for FIMD because
we don't support interlace or anything like that.

Fixes rejection of VGA modes that we can't support due to
register bitwidth limitations, see commit 0f69426afd0a5c7d07d4b2