drm: vkms: Replace hardcoded value of `vkms_composer.map` to DRM_FORMAT_MAX_PLANES

The `map` vector at `vkms_composer` uses a hardcoded value to define its
size.

If someday the maximum number of planes increases, this hardcoded value
can be a problem.

This value is being replaced with the DRM_FORMAT_MAX_PLANES macro.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
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/20220905190811.25024-2-igormtorrente@gmail.com