svga: use the SVGA3D_DEVCAP_MAX_FRAGMENT_SHADER_INSTRUCTIONS query

svga: use the SVGA3D_DEVCAP_MAX_FRAGMENT_SHADER_INSTRUCTIONS query

Instead of a hard-coded 512.  The query typically returns 65536 now.
Fall back to 512 if the query fails as we do for vertex shaders (which
should never happen).

Note that we don't actually enforce this limit in our shaders but it gets
reported via the glGetProgramivARB(GL_MAX_PROGRAM_INSTRUCTIONS_ARB) query.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>