r600g: cache shader variants instead of rebuilding v3

r600g: cache shader variants instead of rebuilding v3

Shader variants are stored in the list, the key for lookup is based on the
states that require different hw shaders - currently it's rctx->two_side (all
gpus) and rctx->nr_cbufs (evergreen/cayman, when writes_all property is set).

v2:
 - use simple list instead of keymap as suggested by Marek on irc
 - call r600_adjust_gprs from r600_bind_vs_shader for r6xx/r7xx
   (r600_shader_select isn't used for vertex shaders currently)

v3:
 - fix call to r600_adjust_gprs - do it after updating current shader

Improves performance for some apps, e.g. FlightGear -
see https://bugs.freedesktop.org/show_bug.cgi?id=50360

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>