drm/amd/display: fix bandwidth validation failure on DCN 2.1

IGT `amdgpu/amd_color/crtc-lut-accuracy` fails right at the beginning of
the test execution, during atomic check, because DC rejects the
bandwidth state for a fb sizing 64x64. The test was previously working
with the deprecated dc_commit_state(). Now using
dc_validate_with_context() approach, the atomic check needs to perform a
full state validation. Therefore, set fast_validation to false in the
dc_validate_global_state call for atomic check.

Cc: stable@vger.kernel.org
Fixes: b8272241 ("drm/amd/display: Drop dc_commit_state in favor of dc_commit_streams")
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>