drm/vmwgfx: Rework screen target page flips v2

Gnome-Shell / Wayland assumes that page-flips can be done on a crtc
regardless of framebuffer size and the crtc position within the
framebuffer.

Therefore rework the screen target code to correctly handle changes in
framebuffer size and content_fb_type. Also make sure that we update
the screen target correctly when the content_fb_type is not
SAME_AS_DISPLAY.

This commit breaks out the framebuffer binding code from crtc_set so it
can be used both from page_flip() and crtc_set() and reworks those
functions a bit to be more robust.

v2: Address review comments by Sinclair Yeh.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>