drm: vkms: Rename `vkms_composer` to `vkms_frame_info`

Changes the name of this struct to a more meaningful name.
A name that represents better what this struct is about.

Composer is the code that do the compositing of the planes.
This struct contains information on the frame used in the output
composition. Thus, vkms_frame_info is a better name to represent
this.

V5: Fix a commit message typo(Melissa Wen).
V6: Fix wrong iosys_map_is_null verification at compose_plane
    (Melissa Wen).

Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Igor Torrente <igormtorrente@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220905190811.25024-3-igormtorrente@gmail.com