drm/amd/display: defer modeset check in dm_update_planes_state

amdgpu_dm_atomic_check() is used to validate the entire configuration of
planes and crtc's that the user space wants to commit.

However amdgpu_dm_atomic_check() depends upon DRM_MODE_ATOMIC_ALLOW_MODESET
flag else its mostly dummy.
Its not mandatory for the user space to set DRM_MODE_ATOMIC_ALLOW_MODESET,
and in general its not set either along with DRM_MODE_ATOMIC_TEST_ONLY.

Considering its importantance, this patch defers the allow_modeset check
in dm_update_planes_state(), so that there shall be scope to validate
the configuration sent from user space, without impacting the population
of dc/dm related data structures.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>