drm/amdgpu: use "irq" in place of "interrupt" in DCE6/8 as in DCE10/11

drm/amdgpu: use "irq" in place of "interrupt" in DCE6/8 as in DCE10/11

"interrupt" becomes "irq" in:
dce_vX_0_set_hpd_interrupt_state()
dce_vX_0_set_crtc_interrupt_state()
dce_vX_0_set_pageflip_interrupt_state()

It is easier when going through the code to just change the DCE number in
the functions' name to find and compare them across DCE versions.

Also, it standardizes function mapping inside a given structure where .set
and .process are both set to functions with a "_irq" suffix.

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>