drm/vkms: Switch to dynamic allocation for CRTC

A specific allocation for the CRTC is not strictly necessary at this
point, but in order to implement dynamic configuration of VKMS (configFS),
it will be easier to have one allocation per CRTC.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117-b4-vkms-allocated-v4-3-8ec8fd21aaf6@bootlin.com


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