drm/i915/gvt: replace the gvt_err with gvt_vgpu_err

gvt_err should be used only for the very few critical error message
during host i915 drvier initialization. This patch
1. removes the redundant gvt_err;
2. creates a new gvt_vgpu_err to show errors caused by vgpu;
3. replaces the most gvt_err with gvt_vgpu_err;
4. leaves very few gvt_err for dumping gvt error during host gvt
   initialization.

v2. change name to gvt_vgpu_err and add vgpu id to the message. (Kevin)
    add gpu id to gvt_vgpu_err. (Zhi)
v3. remove gpu id from gvt_vgpu_err caller. (Zhi)
v4. add vgpu check to the gvt_vgpu_err macro. (Zhiyuan)
v5. add comments for v3 and v4.
v6. split the big patch into two, with this patch only for checking
    gvt_vgpu_err. (Zhenyu)
v7. rebase to staging branch
v8. rebase to fix branch

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>