drm/i915: apply WaEnableVGAAccessThroughIOPort as needed

Currently as the workaround is applied the screen flickers. As a result
we do not achieve seamless boot experience.

Avoiding the issue in the common use-case might be hard, although we can
resolve it for dual GPU setups - when the "other" GPU is primary and/or
outputs are connected to it.

With this I was able to get seamless experience on my Intel/Nvidia box,
running systemd-boot and sddm/Xorg. Note that the i915 driver is within
initrd while the Nvidia one is not.

Without this patch, the splash presented by systemd-boot (UEFI BGRT) is
torn down as the code-path kicks in, leaving the monitor blank until the
login manager starts.

Same issue were reported with plymouth/grub, although personally I
wasn't able to get them to behave on my setup.
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210604154905.660142-1-emil.l.velikov@gmail.com

drivers/gpu/drm/i915/display/intel_vga.c

@@ -29,6 +29,9 @@ void intel_vga_disable(struct drm_i915_private *dev_priv)
 	i915_reg_t vga_reg = intel_vga_cntrl_reg(dev_priv);
 	u8 sr1;
 
+	if (intel_de_read(dev_priv, vga_reg) & VGA_DISP_DISABLE)
+		return;
+
 	/* WaEnableVGAAccessThroughIOPort:ctg,elk,ilk,snb,ivb,vlv,hsw */
 	vga_get_uninterruptible(pdev, VGA_RSRC_LEGACY_IO);
 	outb(SR01, VGA_SR_INDEX);