drm/mgag200: Fix PLL setup for G200_SE_A rev >=4

For G200_SE_A, PLL M setting is wrong, which leads to blank screen,
or "signal out of range" on VGA display.
previous code had "m |= 0x80" which was changed to
m |= ((pixpllcn & BIT(8)) >> 1);

Tested on G200_SE_A rev 42

This line of code was moved to another file with
commit 877507bb ("drm/mgag200: Provide per-device callbacks for
PIXPLLC") but can be easily backported before this commit.

v2: * put BIT(7) First to respect MSB-to-LSB (Thomas)
    * Add a comment to explain that this bit must be set (Thomas)

Fixes: 2dd04094 ("drm/mgag200: Store values (not bits) in struct mgag200_pll_values")
Cc: stable@vger.kernel.org
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013132810.521945-1-jfalempe@redhat.com

drivers/gpu/drm/mgag200/mgag200_g200se.c

@@ -284,7 +284,8 @@ static void mgag200_g200se_04_pixpllc_atomic_update(struct drm_crtc *crtc,
 	pixpllcp = pixpllc->p - 1;
 	pixpllcs = pixpllc->s;
 
-	xpixpllcm = pixpllcm | ((pixpllcn & BIT(8)) >> 1);
+	// For G200SE A, BIT(7) should be set unconditionally.
+	xpixpllcm = BIT(7) | pixpllcm;
 	xpixpllcn = pixpllcn;
 	xpixpllcp = (pixpllcs << 3) | pixpllcp;