drm/i915/gvt: Fix emulated vbt size issue

The emulated vbt doesn't tell its size correctly. According to the
intel_vbt_defs.h, vbt_header.vbt_size should the size of VBT (VBT Header,
BDB Header and data blocks), and bdb_header.bdb_size should be the size
of BDB (BDB Header and data blocks).

This patch fixes the issue and lets vbt provided by GVT-g pass the guest
i915's sanity test.

v2: refine the commit message. (Zhenyu)

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200305131600.29640-1-tina.zhang@intel.com