drm/i915/gvt: give the cmd parser decode_info a const treatment

drm/i915/gvt: give the cmd parser decode_info a const treatment

It doesn't need to be changed, make it const. The string literals should
anyway be referred to as const data.

The following gets moved to rodata section:

0000000000000410 l     O .rodata	0000000000000018 decode_info_mi
0000000000000390 l     O .rodata	0000000000000018 decode_info_3d_media
00000000000003e0 l     O .rodata	0000000000000018 decode_info_2d
0000000000000330 l     O .rodata	0000000000000018 decode_info_mfx_vc
00000000000002e0 l     O .rodata	0000000000000018 decode_info_vebox
0000000000000300 l     O .rodata	0000000000000028 sub_op_vebox
0000000000000360 l     O .rodata	0000000000000028 sub_op_mfx_vc
00000000000003c0 l     O .rodata	0000000000000020 sub_op_3d_media
0000000000000400 l     O .rodata	0000000000000010 sub_op_2d
0000000000000430 l     O .rodata	0000000000000010 sub_op_mi

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>