drm/i915: Do not unmask PSR interruption in IRQ postinstall

No need to unmask PSR interrutpion if PSR is not enabled, better move
the call to intel_psr_enable_source().

v2: Renamed intel_psr_irq_control() to psr_irq_control() (Lucas)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820223325.27490-3-jose.souza@intel.com