drm/msm: gpu: call qcom_mdt interfaces only for ARCH_QCOM

When compile-testing for something other than ARCH_QCOM,
we run into a link error:

drivers/gpu/drm/msm/adreno/a5xx_gpu.o: In function `a5xx_hw_init':
a5xx_gpu.c:(.text.a5xx_hw_init+0x600): undefined reference to `qcom_mdt_get_size'
a5xx_gpu.c:(.text.a5xx_hw_init+0x93c): undefined reference to `qcom_mdt_load'

There is already an #ifdef that tries to check for CONFIG_QCOM_MDT_LOADER,
but that symbol is only meaningful when building for ARCH_QCOM.

This adds a compile-time check for ARCH_QCOM, and clarifies the
Kconfig select statement so we don't even try it for other targets.

The check for CONFIG_QCOM_MDT_LOADER can then go away, which also
improves compile-time coverage and makes the code a little nicer
to read.

Fixes: 7c65817e

 ("drm/msm: gpu: Enable zap shader for A5XX")
Acked-by: Jordan Crouse <jcrouse@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>