iris: Fix value of out-of-bounds accesses for vertex attributes

iris: Fix value of out-of-bounds accesses for vertex attributes

Having VERTEX_BUFFER_STATE.BufferSize greater than the size of
a bound vertex buffer allows shader to read uninitialized vertex
attributes from BO, instead of allowing hardware to return zeroes
on out-of-bounds access.

OpenGL spec "6.4 Effects of Accessing Outside Buffer Bounds" says:

"Robust buffer access can be enabled by creating a context with robust access
 enabled through the window system binding APIs. When enabled, any command
 unable to generate a GL error as described above, such as buffer object accesses
 from the active program, will not read or modify memory outside of the data
 store of the buffer object and will not result in GL interruption or termination.
 Out-of-bounds reads may return values from within the buffer object or zero
 values."

Fixes three webgl tests:
 conformance/rendering/out-of-bounds-array-buffers.html
 conformance2/rendering/out-of-bounds-index-buffers-after-copying.html
 conformance2/rendering/element-index-uint.html

See #1996

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3427>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3427>