vlv_display_irq_postinstall() enables the LPE audio interrupts
regardless of whether the LPE audio irq chip has masked/unmasked
them. Also the irqchip masking/unmasking doesn't consider the state
of the display power well or the device, and hence just leads to
dmesg spew when it tries to access the hardware while it's powered
down.

If the current way works, then we don't need to do anything in the
mask/unmask hooks. If it doesn't work, well, then we'd need to properly
track whether the irqchip has masked/unmasked the interrupts when
we enable display interrupts. And the mask/unmask hooks would need
to check whether display interrupts are even enabled before frobbing
with he registers.

So let's just assume the current way works and neuter the mask/unmask
hooks. Also clean up vlv_display_irq_postinstall() a bit and stop
it from trying to unmask/enable the LPE C interrupt on VLV since it
doesn't exist.

Cc: Takashi Iwai <tiwai@suse.de>
Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170427160231.13337-4-ville.syrjala@linux.intel.comReviewed-by: NTakashi Iwai <tiwai@suse.de>