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 7f819a9ba707e6e905168b00b0f3bf6348e86188)
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>

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