drm/i915: Get rid of ibx_irq_pre_postinstall()

ibx_irq_pre_postinstall() looks totally pointless. We can just
init both SDEIMR and SDEIER at the same time before enabling the
master interrupt. It's equally racy as the other order due
to doing all of this from the postinstall stage with the interrupt
handler already in place. That is, safe with MSI but racy with
shared legacy interrupts. Fortunately we should have MSI on all ilk+.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028213323.5423-20-ville.syrjala@linux.intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>