irqchip: mips-gic: Inline gic_local_irq_domain_map()

The gic_local_irq_domain_map() function has only one callsite in
gic_irq_domain_map(), and the split between the two functions makes it
unclear that they duplicate calculations & checks.

Inline gic_local_irq_domain_map() into gic_irq_domain_map() in order to
clean this up. Doing this makes the following small issues obvious, and
the patch tidies them up:

 - Both functions used GIC_HWIRQ_TO_LOCAL() to convert a hwirq number to
   a local IRQ number. We now only do this once. Although the compiler
   ought to have optimised this away before anyway, the change leaves us
   with less duplicate code.

 - gic_local_irq_domain_map() had a check for invalid local interrupt
   numbers (intr > GIC_LOCAL_INT_FDC). This condition can never occur
   because any hwirq higher than those used for local interrupts is a
   shared interrupt, which gic_irq_domain_map() already handles
   separately. We therefore remove this check.

 - The decision of whether to map the interrupt to gic_cpu_pin or
   timer_cpu_pin can be handled within the existing switch statement in
   gic_irq_domain_map(), shortening the code a little.

The change additionally prepares us nicely for the following patch of
the series which would otherwise need to duplicate the check for whether
a local interrupt should be percpu_devid or just percpu (ie. the switch
statement from gic_irq_domain_map()) in gic_local_irq_domain_map().

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>