media: docs: fix PDF build with Sphinx 1.4

Commit 70b074df ("media: fix pdf build with Spinx 1.6") caused
a regression at Sphinx 1.4 PDF build: although it produces a full
document in batch mode, it returns errors on interactive mode:

	[63]
	Runaway argument?
	{\relax
	! Paragraph ended before \multicolumn was complete.
	<to be read again>
	                   \par
	l.7703 \hline\end{tabulary}

The error seems to be due to some bug at Sphinx PDF output:
when multicolumns is used, it doesn't accept an empty string.

Just removing the :cpan:`1` and replacing by two empty
columns fix the issue.

Fixes: 70b074df ("media: fix pdf build with Spinx 1.6")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/pixfmt-packed-rgb.rst

@@ -35,7 +35,8 @@ next to each other in memory.
       - :cspan:`7` Byte 1
       - :cspan:`7` Byte 2
       - :cspan:`7` Byte 3
-    * - :cspan:`1`
+    * -
+      -
       - 7
       - 6
       - 5
@@ -665,7 +666,8 @@ either the corresponding ARGB or XRGB format, depending on the driver.
       - :cspan:`7` Byte 2
 
       - :cspan:`7` Byte 3
-    * - :cspan:`1`
+    * -
+      -
       - 7
       - 6
       - 5

Documentation/media/uapi/v4l/pixfmt-packed-yuv.rst

@@ -37,7 +37,8 @@ component of each pixel in one 16 or 32 bit word.
       - :cspan:`7` Byte 2
 
       - :cspan:`7` Byte 3
-    * - :cspan:`1`
+    * -
+      -
       - 7
       - 6
       - 5