drm/vmwgfx: Don't destroy Screen Target when CRTC is enabled but inactive

drm_crtc_helper_funcs::atomic_disable can be called even when the CRTC is
still enabled. This can occur when the mode changes or the CRTC is set as
inactive.

In the case where the CRTC is being set as inactive we only want to
blank the screen. The Screen Target should remain intact as long as the
mode has not changed and CRTC is enabled.

This fixes a bug with GDM where locking the screen results in a permanent
black screen because the Screen Target is no longer defined.

Fixes: 7b006203

 ("drm/vmwgfx: Implement virtual crc generation")
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240531203358.26677-1-ian.forbes@broadcom.com