iris: Add render target read entry in binding table

iris: Add render target read entry in binding table

This will be used in next patches for supporting non coherent
framebuffer fetch on Broadwell.

v2: Fix comment (Kenneth Graunke)

v3: 1) Fix a few nits (Caio)
    2) Add comment (Caio)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>