drm/i915/gvt: Change fb_info->size from pages to bytes

fb_info->size is in pages, but some function need bytes when it
is as a parameter. Such as:
a. intel_gvt_ggtt_validate_range(), according to function definition
b. vifio_device_gfx_plane_info->size, according to the comment of
   its definition

So change fb_info->size into bytes.

v2: Keep fb_info->size in real size instead of assinging casted page
    size(zhenyu)
v3: obj->size should be page aligned and delete redundant check(zhenyu)

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