i965: return the fourcc saved in __DRIimage when possible

i965: return the fourcc saved in __DRIimage when possible

When creating a image from a texture, the image's dri_format is
set to the first plane's format, and used to look up for the
fourcc. e.g. for FOURCC_NV12 texture, the dri_format is set to
__DRI_IMAGE_FORMAT_R8, we end up with a wrong entry in function
intel_lookup_fourcc():
   { __DRI_IMAGE_FOURCC_R8, __DRI_IMAGE_COMPONENTS_R, 1,
     { { 0, 0, 0, __DRI_IMAGE_FORMAT_R8, 1 }, } },
instead of the correct one:
   { __DRI_IMAGE_FOURCC_NV12, __DRI_IMAGE_COMPONENTS_Y_UV, 2,
     { { 0, 0, 0, __DRI_IMAGE_FORMAT_R8, 1 },
       { 1, 1, 1, __DRI_IMAGE_FORMAT_GR88, 2 } } },
as a result, a wrong fourcc __DRI_IMAGE_FOURCC_R8 was returned.

To fix this bug, the image inherits the texture's planar_format that
has the original fourcc; Upon querying, if planar_format is set,
return the saved fourcc; Otherwise fall back to the old way.

v3: add a bug description and "cc mesa-stable" tag (Jason)
  remove redundant null pointer check (Tapani)
  squash 2 patches into one (James)
v2: fall back to intel_lookup_fourcc() when planar_format is NULL
  (Dongwon & Matt Roper)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Xiong, James <james.xiong@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>