r600g: avoid shader needing too many gpr to lockup the gpu v2

r600g: avoid shader needing too many gpr to lockup the gpu v2

On r6xx/r7xx shader resource management need to make sure that the
shader does not goes over the gpr register limit. Each specific
asic has a maxmimum register that can be split btw shader stage.
For each stage the shader must not use more register than the
limit programmed.

v2: Print an error message when discarding draw. Don't add another
    boolean to context structure, but rather propagate the discard
    boolean through the call chain.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>