drm/mgag200: Don't write-protect CRTC 0-7 while in mga_crtc_prepare()

The prepare function write-protects several registers that it doesn't
even touch. Removed the related code.

The code for unprotecting registers also clears VINT interrupts. Both
is now done once during initialization.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-2-tzimmermann@suse.de