drm/amd/display: Remove page_fleep_needed function.

This function makes a lot of assumptions to try and deduce
tha an atomic_commit came from FLIP ioctl. Those assumptions
are not explicit contract with DRM API and might lead to wrong
result as the code changes. Instead use drm_atomic_state.allow_modeset
flag which explicitly is set to false only in atomic flip
helper function, otherwise this flag is always true.
P.S When we get to do actual atomic IOCTL this will have to be revised
with the atomic call behavior.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>