gpu: nvgpu: cache gpu clk rate

Cache the rate used in clk_set_rate().
Return that cached rate on clk_get_rate(), don't read from hardware.
This cached rate is used to avoid duplicate requests to clk_set_rate().

Motivation is to support multiple governors for gpu clk.
Reading clock from hardware is unreliable in multi-governor situation.
Relying on hardware clock value could mislead the kernel gpu governor
in its scaling calculations.

Bug 2051688

Change-Id: I43fc056eea6f69fe0889c45640fcb892b658071c
Signed-off-by: Arun Kannan <akannan@nvidia.com>
(cherry picked from commit 7f819a9ba7)
Reviewed-on: https://git-master.nvidia.com/r/1662759
Reviewed-on: https://git-master.nvidia.com/r/1668919
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/linux/platform_gk20a.h

@@ -256,6 +256,9 @@ struct gk20a_platform {
 
 	/* stream id to use */
 	u32 ltc_streamid;
+
+	/* scaling rate */
+	unsigned long cached_rate;
 };
 
 static inline struct gk20a_platform *gk20a_get_platform(