gallium: Always map for READ flag when DISCARD is not set.

gallium: Always map for READ flag when DISCARD is not set.

This prevents the driver from discarding a buffer when the whole buffer
is mapped for writing, but only a portion is effectively written.

This is a temporary fix, because WRITE shouldn't imply DISCARD.

The full fix implies using PIPE_BUFFER_USAGE_DISCARD, throughout
the code, and will go only into master.