i965: Clamp texture buffer size to GL_MAX_TEXTURE_BUFFER_SIZE.

i965: Clamp texture buffer size to GL_MAX_TEXTURE_BUFFER_SIZE.

The OpenGL 4.5 specification's description of TexBuffer says:

"The number of texels in the texture image is then clamped to an
 implementation-dependent limit, the value of MAX_TEXTURE_BUFFER_SIZE."

We set GL_MAX_TEXTURE_BUFFER_SIZE to 2^27.  For buffers with a byte
element size, this is the maximum possible size we can encode in
SURFACE_STATE.  If you bind a buffer object larger than this as a
texture buffer object, we'll exceed that limit and hit an isl assert:

   assert(num_elements <= (1ull << 27));

To fix this, clamp the size in bytes to MaxTextureSize / texel_size.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>