drm/simple-kms: Add drm_simple_encoder_{init,create}()

drm/simple-kms: Add drm_simple_encoder_{init,create}()

This patch makes the internal encoder implementation of the simple
KMS helpers available to drivers.

These simple-encoder helpers initialize an encoder with an empty
implementation. This covers the requirements of most of the existing
DRM drivers. A call to drm_simple_encoder_create() allocates and
initializes an encoder instance, a call to drm_simple_encoder_init()
initializes a pre-allocated instance.

v3:
	* remove drm_simple_encoder_create(); not required yet
	* provide more precise documentation
v2:
	* move simple encoder to KMS helpers
	* remove name argument; simplifies implementation
	* don't allocate with devm_ interfaces; unsafe with DRM

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228081828.18463-2-tzimmermann@suse.de