drm/vkms: Use drm_frame directly

Remove intermidiary variables and access the variables directly from
drm_frame. These changes should be noop.

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
[Louis Chauvet: Applied review from Maíra]
Link: https://patchwork.freedesktop.org/patch/msgid/20241118-yuv-v14-2-2dbc2f1e222c@bootlin.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>