fbdev: sm712fb: fix VRAM detection, don't set SR70/71/74/75

commit dcf9070595e100942c539e229dde4770aaeaa4e9 upstream.

On a Thinkpad s30 (Pentium III / i440MX, Lynx3DM), the amount of Video
RAM is not detected correctly by the xf86-video-siliconmotion driver.
This is because sm712fb overwrites the GPR71 Scratch Pad Register, which
is set by BIOS on x86 and used to indicate amount of VRAM.

Other Scratch Pad Registers, including GPR70/74/75, don't have the same
side-effect, but overwriting to them is still questionable, as they are
not related to modesetting.

Stop writing to SR70/71/74/75 (a.k.a GPR70/71/74/75).
Signed-off-by: NYifeng Li <tomli@tomli.me>
Tested-by: NSudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Cc: <stable@vger.kernel.org>  # v4.4+
Signed-off-by: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/video/fbdev/sm712fb.c

@@ -1146,7 +1146,9 @@ static void sm7xx_set_timing(struct smtcfb_info *sfb)
 		/* init SEQ register SR30 - SR75 */
 		for (i = 0; i < SIZE_SR30_SR75; i++)
 			if ((i + 0x30) != 0x30 && (i + 0x30) != 0x62 &&
-			    (i + 0x30) != 0x6a && (i + 0x30) != 0x6b)
+			    (i + 0x30) != 0x6a && (i + 0x30) != 0x6b &&
+			    (i + 0x30) != 0x70 && (i + 0x30) != 0x71 &&
+			    (i + 0x30) != 0x74 && (i + 0x30) != 0x75)
 				smtc_seqw(i + 0x30,
 					  vgamode[j].init_sr30_sr75[i]);