drm/mgag200: Add module parameter to pin all buffers at offset 0

For hardware that does not interpret the startadd field correctly,
add the module parameter 'hw_bug_no_startadd', which enables the
workaround.

v3:
	* style and typo fixes
v2:
	* ask user for feedback if the option is active

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: John Donnelly <john.p.donnelly@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126101950.11989-1-tzimmermann@suse.de