Enable vrs2x2 coarse shading if flat shading as per
idea and guidance given by Marek.

is_flat_shading variable in struct si_shader_info is set
based on the data from gather_intrinsic_info() function
and struct si_state_rasterizer. If is_flat_shading_variable
is set, then in function si_emit_db_render_state() vrs2x2
shading is enabled in hardware.

v2: Fix review comments from Pierre-Eric. Code optimizations.
v3: Fix indentation style issue.
v4: Fix review comments from Marek. Fixed logical issue pointed
    by Marek where info->is_flat_shading variable can be corrupted
    and other code cleanup.
v5: Make the code compact as suggested by Pierre-Eric.
v6: Fix new review comments from Marek.
v7: use info->uses_interp_color variable fix from Marek.
v8: Fix coding style comment from Marek.
v9: Add uses_fbfetch_output check as suggested by Marek.

Signed-off-by: Yogesh Mohan Marimuthu <yogesh.mohanmarimuthu@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <!8161>