drm/omap: fix: disable encoder before destroying it

Currently, an encoder is disabled only when an apply work is queued for the
corresponding crtc. This works fine for the paths where userspace explicitly
disables crtc, this results in disabling the omapdss device in the crtc's
pre_apply function.

However, when the omapdrm module is removed, there is no work queued to ensure
that the encoder is disabled. This can result in an enabled omapdss device when
removing omapdrm. omapdss is left in an inconsistent state, and that prevents us
from using that omapdss device being used again.

Disable enabled encoders in omap_encoder_destroy, we could consider a better
place for doing this later.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>