drm/i915/gvt: combine access to consecutive guest context pages

IOVA(GPA)s of context pages are checked and if they are consecutive,
read/write them together in one intel_gvt_hypervisor_read_gpa() /
intel_gvt_hypervisor_write_gpa().

Signed-off-by: Yan Zhao <yan.y.zhao@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/20200415035827.26476-1-yan.y.zhao@intel.com