drm/amdgpu: Move existing pflip fields into separate struct

[Why&How]
To refactor DM IRQ management, all fields used by IRQ is best moved
to a separate struct so that main amdgpu_crtc struct need not be changed
Location of the new struct shall be in DM

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>