drm/vmwgfx: Refactor cursor handling

Refactor cursor handling to make the code maintainable again. Over the
last 12 years the svga device improved support for virtualized cursors
and at the same time the drm interfaces evolved quite a bit from
pre-atomic to current atomic ones. vmwgfx only added new code over
the years, instead of adjusting/refactoring the paths.

Export the cursor plane handling to its own file. Remove special
handling of the legacy cursor support to make it fit within the global
cursor plane mechanism.

Finally redo dirty tracking because memcmp never worked correctly
resulting in the cursor not being properly updated in the guest.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250307125836.3877138-2-zack.rusin@broadcom.com