radeonsi: make gl_SampleMaskIn = 0x1 when MSAA is disabled

radeonsi: make gl_SampleMaskIn = 0x1 when MSAA is disabled

gl_SampleMaskIn is 1 when R_028BE0_PA_SC_AA_CONFIG is 0, so this commit rework the conditions
controlling this register.

Before it was set if the sctx->framebuffer had a sample count > 1.

Now we still require this condition, but we also need either:
  - GL_MULTISAMPLE to be enabled
  - to be executing an operation that doesn't depends on GL state using u_blitter.

This fixes the arb_sample_shading/sample_mask piglit tests on radeonsi.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>