drm/vc4: fix typo

Replace 'pack' with 'back'.

Fixes: c8b75bca ("drm/vc4: Add KMS support for Raspberry Pi.")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023085929.1445594-1-dario.binacchi@amarulasolutions.com

drivers/gpu/drm/vc4/vc4_regs.h

@@ -588,7 +588,7 @@ enum {
 # define VC4_HDMI_HORZA_HAP_MASK		VC4_MASK(12, 0)
 # define VC4_HDMI_HORZA_HAP_SHIFT		0
 
-/* Horizontal pack porch (htotal - hsync_end). */
+/* Horizontal back porch (htotal - hsync_end). */
 # define VC4_HDMI_HORZB_HBP_MASK		VC4_MASK(29, 20)
 # define VC4_HDMI_HORZB_HBP_SHIFT		20
 /* Horizontal sync pulse (hsync_end - hsync_start). */