i965: glClearBuffer() should only clear a single buffer.

i965: glClearBuffer() should only clear a single buffer.

glClearBuffer() is currently clearing all active draw color buffers (all
buffers that have not been set to GL_NONE when calling glDrawBuffers) instead
of only clearing the one it receives as parameter. Altough brw_clear()
receives a bit mask indicating the color buffers that should be cleared,
this mask is ignored when calling brw_blorp_clear_color().

This was breaking the 'fbo-drawbuffers-none glClearBuffer' piglit test.

The patch provides the bit mask to brw_blorp_clear_color() so it can limit
clearing to the color buffers present in the mask.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76832
Reviewed-by: Eric Anholt <eric@anholt.net>