drm/vkms: Switch to dynamic allocation for encoder

A specific allocation for the encoder 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 encoder.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117-b4-vkms-allocated-v4-2-8ec8fd21aaf6@bootlin.com


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