media: exynos4-is: make array 'cmd' static, shrinks object size

Don't populate the const read-only array 'cmd' on the stack but instead
make it static. Makes the object code smaller by 38 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   4950	    868	      0	   5818	   16ba	fimc-is-regs.o

After:
   text	   data	    bss	    dec	    hex	filename
   4824	    956	      0	   5780	   1694	fimc-is-regs.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>