drm/vmwgfx: Don't look at state->allow_modeset

That's purely for the uapi layer to implement the ALLOW_MODESET flag.

Drivers should instead look at the state, e.g. through
drm_atomic_crtc_needs_modeset(), which vmwgfx already does. Also remove
the confusing comment, since checking allow_modeset is at best a micro
optimization.

v2: Rebase

Reviewed-by: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190520223500.6032-1-daniel.vetter@ffwll.ch