drm/mgag200: Split DPMS function into helpers

Of the DPMS code, only ON and OFF states are used. Simplify mode setting
by moving both into separate functions and removing the rest.

The original code busy waited in the middle of updating the screen state
in SEQ1. To simplify the procedure, the new code busy waits first and then
updates SEQ1 in one chunk.

The DPMS code also set the LUT before enabling the screen. The patch moves
this code into the simple-display pipe's enable function.

v2:
	* comment on SEQ1 updates in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-5-tzimmermann@suse.de