svga: fix format for screen target

svga: fix format for screen target

This patch revises the fix in commit 606f13afa31c9f041a68eb22cc32112ce813f944
to properly translate the surface format for screen target.
Instead of changing the svga format for PIPE_FORMAT_B5G6R5_UNORM
to SVGA3D_R5G6B5 for all texture surfaces, this patch only restricts
SVGA3D_R5G6B5 for screen target surfaces. This avoids rendering
failures when specify a non-vgpu10 format in a vgpu10 context with
software renderer.

Fixes piglit failures spec@!opengl 1.1@draw-pixels,
                      spec@!opengl 1.1@teximage-colors gl_r3_g3_b2
                      spec@!opengl 1.1@texwrap formats

Tested Xorg with 16bits depth.
Also tested with MTT piglit, MTT glretrace.

Reviewed-by: Brian Paul <brianp@vmware.com>