drm/exynos: kill pipe field from drivers contexts

Since possible_crtcs are set by Exynos core helper pipe fields have no
raison d'etre. The only place it was used, as a hack, is
fimd_clear_channels, to avoid calling drm_crtc_handle_vblank, but DRM core
has already other protection mechanism (vblank->enabled), so it could be
safely removed.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>