intel: in intel_context struct use typedef for sarea struct

intel: in intel_context struct use typedef for sarea struct

Using drm_i915_sarea_t instead of struct drm_i915_sarea seems to be
a common standard now, therefore fix it also in intel_context
structure. Additionally this silences a compiler warning:

intel_swapbuffers.c: In function `intelFixupVblank':
intel_swapbuffers.c:48: warning: initialization from incompatible pointer type

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>