drm/vkms: allow the primary plane to be positioned

Before commit bc0d7fde ("drm: vkms: Supports to the case where
primary plane doesn't match the CRTC"), the composition was executed
on top of the primary plane. Therefore, the primary plane needed to cover
the entire CRTC. After commit bc0d7fde, this is no longer necessary,
as the composition is now executed on top of the CRTC.

Then, allow the primary plane to be positioned in such a way that it
doesn't cover the entire CRTC.

This patch was tested with the vkms IGT testlist and all tests passed
successfully. Moreover, the test
igt@kms_universal_plane@universal-plane-pageflip-windowed-pipe-A
used to fail and now is passing.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230324164226.256084-3-mcanal@igalia.com