drm/vkms: rename cursor to plane on ops of planes composition

Generalize variables and function names used for planes composition
(from cursor to plane), since we will reuse the operations for both
cursor and overlay types.

No functional change.

v4:
- use better names for functions of plane composition (Daniel)

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/ef3249c9ac3a08b7079894f9f1ccf70960194ec7.1619250933.git.melissa.srw@gmail.com