drm/encoder: Add macro drmm_plain_encoder_alloc()

This performs the same operation as drmm_encoder_alloc(), but
only allocates and returns a struct drm_encoder instance.

v4: Rename macro drmm_plain_encoder_alloc() and move to
    <drm/drm_encoder.h>. Since it's not "simple" anymore it
    will now take funcs/name arguments as well.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210327115742.18986-3-paul@crapouillou.net