gallium: introduce get_device_vendor() entrypoint for pipes

This will be needed by Clover to return the correct information
to CL_DEVICE_VENDOR info queries.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/gallium/docs/source/screen.rst

@@ -462,6 +462,12 @@ get_vendor
 
 Returns the screen vendor.
 
+get_device_vendor
+^^^^^^^^^^^^^^^^^
+
+Returns the actual vendor of the device driving the screen
+(as opposed to the driver vendor).
+
 .. _get_param:
 
 get_param