drm/amdgpu: update the shader to clear specific SGPRs

Add shader codes to explicitly clear specific SGPRs, such as
flat_scratch_lo, flat_scratch_hi and so on. And also correct the
allocation size of SGPRs in PGM_RSRC1.

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>