drm/vram: Support scanline alignment for dumb buffers

Adding the pitch alignment as an argument to
drm_gem_vram_fill_create_dumb() allows to align scanlines to certain
offsets. A value of 0 disables scanline pitches.

v3:
	* only do power-of-2 test if pitch_align given; fails otherwise
	* mgag200: call drm_gem_vram_fill_create_dumb() with pitch_align
v2:
	* split of patch from related hibmc changes
	* test if scanline pitch is power of 2

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203083819.6643-4-tzimmermann@suse.de