freedreno/ir3: shader variant cache

freedreno/ir3: shader variant cache

Cache that maps gallium hwcso (in this case, 'struct ir3_shader') plus
shader variant key to a generation specific state object.

This could eventually replace the linked list of shader variants, but
for now it lets us re-use the work currently done in fdN_program_emit()

Signed-off-by: Rob Clark <robdclark@gmail.com>