gallium: add PIPE_CAP_INVALIDATE_BUFFER

It makes sense to re-use pipe->invalidate_resource for the purpose of
glInvalidateBufferData, but this function is already implemented in vc4
where it doesn't have the expected behavior. So add a capability flag
to indicate that the driver supports the expected behavior.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/docs/source/screen.rst

@@ -301,6 +301,8 @@ The integer capabilities:
   alignment for pipe_shader_buffer::buffer_offset, in bytes. Maximum
   value allowed is 256 (for GL conformance). 0 is only allowed if
   shader buffers are not supported.
+* ``PIPE_CAP_INVALIDATE_BUFFER``: Whether the use of ``invalidate_resource``
+  for buffers is supported.
 
 
 .. _pipe_capf: