drm/msm/gpu: Remove hardcoded interrupt name

Every GPU core only has one interrupt so there isn't any
value in looking up the interrupt by name. Remove the name (which
is legacy anyway) and use platform_get_irq() instead.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>