drm/i915/gvt: Delete redundant error message in fb_decode.c

Much error message exist in host dmesg when guest boot up with local
display enabled.
[  167.680011] gvt: vgpu 1: invalid range gmadr 0x0 size 0x0
[  167.680013] gvt: vgpu 1: invalid gma address: 0

The second error line duplicate with the first error line, so this
patch remove this redundant error message and make the next error
message much clearer.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>